I don't go out of my way to read sexually explicit fanfic, but I don't avoid a story just because of it. And as this is a modern story with middle-upper-class scientific/medical sorts, sex is usually preceded by a discussion of condoms. Which I applaud - good example, you know, plus in keeping with people who are ultimately as sensible as John and Watson (not necessarily in combination - but one or the other is usually involved in the conversation) are.
Only... when it's Sherlock he typically insists condoms aren't necessary, because he's been involved in some sort of horrible accident at the morgue and had to have a full medical work-up so knows he's clean. And by involved, I usually mean he's blown up a corpse. This means he's (a) sure he hasn't had sex in the meantime, and (b) is causing corpses to blow up on a semi-regular basis. I've read maybe ten stories where Sherlock has the Talk, with at least seven or eight involving the Incident at St. Bart's. I'm beginning to think blowing up corpses is how he dealt with boredom before John's illegal handgun opened up other options, and that's why Molly was surprised he only needed to see the feet in "Blind Banker." She was expecting to need a HazMat crew.
Both those conditions, you know, are completely 100% plausible.
And that, ladies and gentlemen, is our show.