Sex is big in novels at the moment. I'm sure you've noticed. Every time I go into WHSmith there's somebody brazenly standing in front of the huge 50 Shades section. And it's going to get worse as the massive band wagon rolls into town and bookshops have to clear away all the vampire books to make room.
If you've not read Echo (and why not?), you'll know there's sex. Twice. E winces and skips over those parts, the crowd at work read it aloud in the office, fortunately whilst I'm not there although they tell me about it.
There's an unspoken presumption that sex scenes in books are autobiographical. Hand on heart there are no polka dot pants in the house. I've no idea whether EL James did theoretical or practical research. Thnk what you like but remember it's a novel of fiction :-)
As I've said before, Echo was written with the intention that nobody would ever read it, but it was still very hard to write. And it will be even harder with Clarke because I know that it will have an audience, and a fairly merciless one at that.
Because everybody is too busy taking the piss, it's almost impossible to get any constructive feedback about those scenes. I read the first too chapters of 50 Shades and they're terrible, so I can only presume that the sex (page 115 onwards, apparently) is truly badly written as well. Is it possible to write a "good" sex scene, or do writers leave their critical facilities at the door as well?
I've never actually questioned why I left those parts in or what they add to the book, I just wrote them. I suppose that I could leave the scene with the closing of the door.
But where would be the fun in that?