Fans of BBC Sherlock wouldn't be the first to interpret the relationship between Holmes as Watson as something more than friends. In the original stories written by Sir Arthur Conan Doyle, Holmes was a bachelor, which at the time was often code for homosexual. Fan nekosmuse analyzes the original text even further.
"It was in the year '95 that a combination of events, into which I need not enter, caused Mr. Sherlock Holmes and myself to spend some weeks in one of our great University towns." -Sir Arthur Conan Doyle, The Three Students
Though modern readers might be cinfused by this passage, at the time people were very ware of the salacious case ahainst Oscar Wilde, who had been accused and convicted of sodomy, which was a crime associated with severe penalties. So, while it would not have been possible for Doyle to write Holmes as textually gay, the original author did hint at the possibility. The BBC adaptation has taken some liberties in reimagining the characters, since they have chosen a modern setting and forensic science has evolved since Doyle was writing. But the characters themselves have essentially stayed the same. But what might have been subtext in the original stories could safely be text today. Are fans seeing something that just isn't there?
Popular among a subset of BBC Sherlock fans is The Johnlock Conspiracy (TJLC). This group of fans sees a subtextual romantic attraction between John and Sherlock, and suspects that the show will eventually develop their platonic relationship into a romantic one. In the most recent season, John gets married (as he did in the original stories). Sir Arthur Conan Doyle introduced John's wife, only to have her die shorly thereafter, giving John a reason to move back in with Sherlock. Fans suspect that the BBC showrunners will follow a similar plot, since they have thus far adhered to the basic structure that the original stories provided for them (with a few significant exceptions). It does seem a little far fetched that a modern adptation would introduce a compelling character like Mary only to kill her off though, and not just because it wouldn't be in very good taste...
The rights to Conan Doyle's estate are still owned by his desvendants. While some of the stories (and the characters) have entered into the public domain, the final stories still have a few years left under copyright. In the meantime, the Doyle estate has given its stamp of approval to the BBC, Guy Ritchie, and CBS adaptations. While legally the estate should no longer have any legal standing regarding stories written before 1920, it doesn't seem like a good idea to upset them either. Here's what they had to say on the subject when Robert Downey Jr. joked about the possibility of Holmes' homosexuality:
"I hope this is just an example of Mr Downey's black sense of humor. It would be drastic, but I would withdraw permission for more films to be made if they feel that is a theme they wish to bring out in the future. I am not hostile to homosexuals, but I am to anyone who is not true to the spirit of the books."
Sounds a lot like "I'm not homophobic, I have a gay best friend" to me. Maybe I'm just being oversensitive. Since those comments were made in 2010, years before the Supreme Court ruled in favor of the character belonging in the public domain, it seems like the BBC adaptation wouldn't have been able plan ahead for a Big Gay Reveal later on (the first season had not yet aired, so there was no guarantee the show would even be successful enough to merit further seasons). That's not to say the subtext isn't there. It merely suggests that it will remain that way.