My understanding of real-world M/M relationships is that anal sex is not nearly as common as it's portrayed to be in porn or media in general. I'd love to see a story where virgin!Sherlock (or possibly, just pretty inexperienced Sherlock) and John begin a relationship and, although the idea of it doesn't really appeal to him (Sherlock), Sherlock expects that John will want anal sex. John, who has had sexual relationships with men (or at least one man) in the past, but has never had or wanted to have anal sex, is confused about why Sherlock seems so insistent about the idea (not in verbally saying that he wants it, but in his actions). Cue discussion, maybe some hurt/comfort, explanations of the difference between porn and reality, but eventually a happy ending!