Ah, sex scenes. You've read the why's. One assumes that if we tried to explain the how's this would be a different kind of blog...but riffing off of KAK's excellent observation that in novels no one farts during sex, can we agree that sex scenes in romance novels have remarkably little resemblence to reality?
When was the last time you read a BDSM where the hero is subjecting the luscious heroine to all sorts of sexual games until she's begging to be taken only to have him say, "You'll take what I give you and like it because I'm the master here! And also, the Viagra hasn't kicked in yet."
Have you ever read a romance sex scene wherein the heroine *wasn't* capable of achieving orgasm? I'd had no idea that some women's bodies simply don't work that way. Women's health information sources suggest that 10% of women are unable to reach orgasm, yet we don't see that in our idealized sex scenes.
I'm not suggesting that fictionalized sex scenes should depict the sometimes messy, amusing and occassionaly mortifiying realities of sex. It is fiction we're all writing, after all, and these idealized, intimate scenes serve purposes specific to each story.
For most romance stories, the physical acts aren't as interesting (to me) as the emotional components driving them. In fact, my favorite part of a sex scene isn't the sex, it's all the lead up to the point where the characters involved decide the reward of giving in is greater than the risk. Done well, there's a great psychological dance between partners, replete with all kinds of discovery and give and take - as much as in the actual sex scene itself - because the psychological dance leading up to intercourse is a kind of test bed.
Get the steps of *that* dance right and you might get to dance between the sheets...too many psychological missteps, though, and neither partner will need a cold shower.
Image source: http://en.wikipedia.org/wiki/Modern_dance