It can be done, but it's tricky. Really, I think it is the easiest thing to mess up in writing, and for that reason it isn't a bad idea to at least practice writing this sort of thing (or do field research if you are lucky)

I think the problem is when the story drops into fantasizing. Sure, if you read enough writing from a person who includes sex or/and romance, you can probably pick out their fantasies and fetishes even if they don't intend it, but some people get carried away and suddenly you end up with a "Dear Penthouse" type piece.

And some people write it like it is porn. That is against forum rules of course, but it also looks like shit. The more biological it sounds, the less effective it is. Nobody ever needs a physical play by play.