The first Sherlock Holmes story written by Sir Arthur Conan Doyle, A Study in Scarlet, was published in 1887 and was set in 1881. This, of course, was more than 70 years before the sexual revolution that began in the Western world in the mid-1960's. Even so, there has still been much speculation in regard to the sexual orientation of the famous detective, as well as that of his longtime friend and colleague, Dr. John Watson, who is the narrator of most of the Holmes stories.
In specific regard to Sherlock Holmes, he has been thought to be heterosexual, homosexual, asexual, or some combination thereof, though nothing is ever stated in canon that explicitly confirms any of them. However, due to the rather unusual relationship between Holmes and Dr. Watson, and the fact that Holmes never married, the strongest case tends to be that the two men were in some sort of romantic relationship, ignoring the various marriages that Watson himself had over the 17 years that he chronicled Holmes' cases.
In 2010, BBC aired a new series called Sherlock, created by Steven Moffat and Mark Gatiss, which brought the detective and his doctor into modern-day London, 50 years after the beginning of the sexual revolution. This, of course, brings with it the opportunity to touch more directly on the subject of sexuality than Doyle was able to in the late 19th and early 20th century, when he was writing. The following will compare how both portray sexuality, and why readers and viewers have come to the conclusions that they have.
In both A Study in Scarlet and A Study in Pink, the modern version of the introductory novel, Sherlock Holmes and Dr. Watson meet through a mutual acquaintance who happens to know that both of them are unable to afford London housing on their own and are in need of a flatshare. For two unmarried men in 19th century London, this may have been somewhat ordinary, but the first thing the landlady, Mrs. Hudson, says to the pair in the 2010 series is, "There's another bedroom upstairs if you'll be needing two," to which Dr. Watson responds, "Of course we'll be needing two." Mrs. Hudson seems to take this as a sign of embarrassment, and continues, "Oh, don't worry, there's all sorts around here. Mrs. Turner, next door, has got married ones." And later, as Holmes dashes off to a crime scene, she comments to Watson, "My husband was just the same. But you're more the sitting down type, I can tell." And even later, in the third episode of the series, The Great Game, upon hearing Holmes and Watson having an argument, she asks, "Have you two had a little domestic?"
Though Mrs. Hudson doesn't know Watson at all at this point, she is at least an acquaintance of Holmes, who had ensured that her presumably abusive husband was sentenced to life in prison on a murder charge, and still seems to assume that Watson is his boyfriend, though in a later episode she admits that she has no idea if Holmes has ever been in any sort of relationship. She is not, however, the only acquaintance of Holmes who makes assumptions. Later on in the same episode, Holmes and Watson have a stakeout at a restaurant across the street from a crime scene. Their waiter, who Holmes got off of a murder charge, insists on referring to Watson as Holmes' date, despite Watson's protests otherwise. And in The Great Game, Sally Donovan, a Scotland Yard Police Officer, shortly states that "opposite attract" in regard to how normal Watson seems and how much of a "freak" Holmes is. In the same episode, in a standoff against Moriarty, Holmes' archenemy, Watson offers to give up his life so that Sherlock can get away, on which Moriarty comments: "Oh, he's very sweet. I can see why you [Holmes] keep him around." Even Holmes' own brother Mycroft, who, when he finds out about Dr. Watson in A Study in Pink comments, "Since yesterday you've moved in with him and now you're solving crimes together. Might we expect a happy announcement by the end of the week?" And in The Great Game: "Sherlock's business seems to be booming since you and he became... pals," the extra emphasis on "pals" seeming to imply a lot more than that.
While it is rather curious that the first thing people assume when they see Holmes with another man is that that man is his boyfriend, it doesn't necessarily confirm anything. At another point in A Study in Pink, Watson asks Holmes if he has a girlfriend, to which Holmes responds, "No, not really my area." And then, when Watson asks if he has a boyfriend, Holmes again responds in the negative. "Right, okay. You're unattached like me. Right. Good," is Watson's reaction to this, which causes Holmes to look at him rather strangely for a few seconds before saying, "John, I think you should know that I consider myself married to my work and, while I'm flattered by your interest, I'm not really looking for any-" at which point Watson cuts him off by saying that that wasn't what he meant. The viewer could get two things from this. The first being that Holmes is at least somewhat asexual, though this to some extent may be by choice, as he probably feels a relationship would interfere with his work. So, again, this does not confirm anything. The second thing would be that Sherlock Holmes, who is the master of deduction, and can tell everything about a person just by glancing at them, thinks Dr. Watson is gay. Though it should be mentioned that Holmes does, on occasion, get things wrong.
Of course, in A Study in Scarlet, none of this commentary occurs. In fact, of the few acquaintances mentioned in this novel, only Stamford, who is the one to introduce Holmes and Watson, makes any sort of comment about Holmes himself, and it's as a warning to Watson before he meets Holmes: "You don't know Sherlock Holmes yet. Perhaps you would not care for him as a constant companion... he is a little queer in his ideas." Curiously, no such comment is made by the modern day Stamford, who has very few lines, but seems to be of the impression that Watson will find Holmes amusing, though it's never explained why that is.
Not surprisingly, however, minor character commentaries are not the only references to the sexualities of Holmes and Watson that are given. In the second episode of the 2010 series, The Blind Banker, when Watson mentions to Holmes that he's going on a date with a woman from work named Sarah, Holmes immediately suggests that he take her to a Chinese circus, which Holmes ends up inviting himself on, as it really has something to do with the case he's working on-though he spends half of the evening following Watson and Sarah around and interrupting them every time they start to get intimate. In the end, Sarah ends up almost getting killed, and while she does show up in later episodes, it's only as a friend of Watson's. Unfortunately, this is not the only time in the series that Holmes seems to put a damper on Watson's relationships. In the first episode of the second season of Sherlock, A Scandal in Belgravia, during a Christmas party, Holmes calls Watson's girlfriend by every name that isn't actually hers, despite the fact that he seemingly has no trouble remembering anyone else's name, which she seems rather offended by, considering that Holmes is supposed to be Watson's best friend. This leads up to later in the episode, when Watson has to run off to help Holmes, her saying, "You know, you're a great boyfriend. Sherlock Holmes is a very lucky man." Needless to say, this relationship didn't last much longer.
The episode The Blind Banker is loosely based on the Sir Arthur Conan Doyle short story The Adventure of the Dancing Men, which was set in 1898, 17 years after A Study in Scarlet. Of course, there are many stories in-between, and by this time, Holmes and Watson know each other quite well. Watson, by this point, has also been married at least twice, though there are theories that he may have been married at least six times, and once more after this in 1902, based on when Watson is and isn't living at 221B Baker Street with Holmes. It is never clear as to what happens to any of these women, though it has to be assumed that they died, as it was not legal to divorce in England at the time. After all of these marriages, and during his marriages when his wife is away, however, Watson always ends up back with Sherlock Holmes, which Holmes always seems happy about, and on several occasions it's rather implied that Holmes doesn't like being without Watson. In the short story The Five Orange Pips, published in 1891 and set in 1887, Holmes states that he "will do nothing serious without [his] trusted comrade and biographer at [his] elbow." And in the 1891 published and 1888 set A Scandal in Bohemia, in which Watson simply comes over for a visit, Holmes says that he is "lost without [his] Boswell," "Boswell" referring to an "assiduous and devoted admirer, student, and recorder of another's words and deeds," though Holmes probably means it in a somewhat more endearing way than the definition would imply. This latter line inspires the 2010 The Great Game Holmes line, "I'd be lost without my blogger," as in the modern series Watson records all of Holmes' cases on his blog.
While all of these things could obviously be indications of a romantic relationship between Holmes and Watson, the most convincing moments in both the original stories and the modern series are when they show genuine concern for one another. This is particularly important for Holmes' character, as it very rarely seems like he truly cares about anyone. For example, during The Great Game, during the scene in which Holmes has a standoff against Moriarty, Watson has previously been strapped into a coat weighed down with explosives, a message from Moriarty. When Moriarty leaves, however, Holmes wastes no time in ripping the coat off of Watson and tossing it as far away from him as possible, repeatedly asking if Watson is all right. Watson jokingly responds to this by saying, "I'm glad no one saw that... You ripping my clothes off... People might talk." Similarly, in the 1912 short story The Adventure of the Bruce-Partington Plans, set in 1895, when Holmes asks Watson to accompany him to break into the house of a suspected criminal, Holmes shows great concern for the fact that not only could Watson have gotten hurt, but also arrested. Watson, in his narration, comments, "For a moment I saw something in his eyes which was nearer to tenderness than I had ever seen."
Of course, as previously mentioned, besides the speculation around a romantic relationship occurring between Holmes and Watson, there are also theories that Holmes may have been either heterosexual or asexual, and that there was no relationship between Holmes and Watson. The greatest argument for these theories comes from A Scandal in Belgravia, which is based on the 1891 A Scandal in Bohemia, which was set in 1887. In both of these stories, Holmes is hired to retrieve compromising photographs from a woman named Irene Adler, who is perhaps the most famous women in the Holmes stories, as she was the only one to ever outsmart Holmes. Not only was she very clever, but also she was a courtier, a singer, and as she did not marry until her late twenties, a "spinster". Because of this, she was considered to be a very liberal woman for her time, and keeping with this, the modern series made her a dominatrix and a lesbian, though the latter does not keep her from being a romantic interest of Holmes. However, when Mycroft, who is the one to hire Holmes on behalf of the Royal Family, informs Holmes of Adler's profession, Holmes seems to not know what it is, and Mycroft continues, "Don't be alarmed; it's to do with sex," to which Holmes responds, "Sex doesn't alarm me." Mycroft then laughs and asks, "How would you know?" This implies that Mycroft thinks Holmes is a virgin, despite previous comments in which he seems to assume that Watson is Holmes' boyfriend. Though Moriarty, who has also eluded to the fact that he things Holmes and Watson are a couple, coined Holmes with the nickname "The Virgin", which Adler mentions later on in the episode. This is similar, nevertheless, to how Adler herself, though she spends most of the episode flirting with Holmes, still makes several comments about Holmes and Watson being a couple. For instance, when Holmes and Watson make their way into Adler's house by pretending to be a pickpocketed priest and an eyewitness who need to call the police, Adler already knows who they are, and as Holmes had made Watson punch him to add effect, Adler comments, "Somebody [Watson] loves you. If I had to punch that face, I'd avoid your nose and teeth, too." This also leads to probably one of the most interesting bits of dialogue in the entire series, in which, after Adler fakes her own death-which has Holmes in a rather depressed, heartbroken state-she confronts Watson when he is alone and informs him that she's still alive. This makes Watson furious, as he knows how upset Holmes has been, particularly because Adler had spent so much time prior to her "death" corresponding with Holmes via text message, which Holmes seemed to enjoy. Adler then remarks that he couldn't have enjoyed it that much, as when she tried to flirt with him, he would never respond, which Watson finds surprising. He says, "Sherlock replies to everything. He will literally outlive God trying to get the last word," to which Adler asks, "Does that make me special? Are you jealous?" And possibly for the hundredth time in the series, Watson responds with, "We're not a couple." Not surprisingly, Adler then says, "Yes, you are." Frustrated, Watson replies with, "Who the hell knows about Sherlock Holmes, but for the record, I'm not actually gay." Irene smiles. "Well, I am," she says. "Look at us both," implying that perhaps love has nothing to do with sexual orientation at all, but just with the people themselves.
Amusingly, however, while Adler is busy thinking Holmes and Watson are a couple, Watson is equally busy thinking the same of Holmes and Adler. During a later scene, after Holmes has found out that Adler is alive, the two are starring at each other rather intently, which Watson awkwardly breaks into by saying, "Hamish... John Hamish Watson. Just if you were looking for baby names." And Mycroft, who has previously accused his brother of being homosexual as well as asexual, assumes that the crush Holmes has on Adler is the reason that he blindly broke military codes for her; if he hadn't been trying so hard to impress her, he would have realized that she was working for Moriarty and cracking any sort of code for her probably wasn't such a good idea.
Toward the end of the episode, though, Holmes again eludes to the fact that he's at least partially asexual by choice when, after finding out that the password on Adler's phone, which holds the information that Holmes was originally hired to retrieve, is his name, he says, "I've always assumed love was a dangerous disadvantage. Thank you for the final proof." Even so, Holmes still dubs her with the title "the woman", which, in a conversation between Mycroft and Watson, Mycroft calls a salute. Watson, however, disagrees, admitting that Holmes "is not like that. He doesn't feel things in that way," which is a summation of the opening dialogue from A Scandal in Bohemia, where 19th century Watson acknowledges the same thing. Mycroft then disagrees further by saying, "My brother has the mind of a scientist or a philosopher, yet he elects to be a detective. What might we deduce about his heart?" This, finally, may explain why his comments about Holmes' sexuality are so irresolute.
All of that said, at the very end of the episode, once Adler has disappeared again, Holmes asks to keep her phone, which had been in police custody. This may be because he knows where she is and wants to give it back to her, and not simply as a reminder of her, but either way, it shows that he at least cared about her on some level, romantic or not. This is similar to how in A Scandal in Bohemia, Holmes asks to keep the photograph of her that he'd been hired to retrieve, even though in this case, he really doesn't know where she ran off to.
At the end of it all, neither viewer nor reader can be entirely certain of the sexuality of Sherlock Holmes, or of Dr. Watson. Were they in a relationship? Maybe. There's certainly evidence to back it up. Did Holmes have a crush on Irene Adler? Perhaps. There's evidence of that, too. Or, as tends to be the least liked theory, was Holmes really asexual, as several comments from both Dr. Watson and Holmes himself would suggest? Possibly. If you look to hard into any of these theories, arguments can surely be found. Though, as Sir Arthur Conan Doyle, the original mind behind Sherlock Holmes, passed away some years ago, we'll probably never get a definite answer.