In my games, it is muchly of the fade-to-black approach, though special circumstances sometimes require more detail. We have little interest in RPing these scenes either with each other or in front of others; sex is either attached to romance or used to establish a character's personality. Most in my group play characters with libidos normal for people who risk their lives every day, with varying preferences for romantic encounters and... financially-motivated ones.
We have one combat-oriented powergamer who simply doesn't think about such things, but we generally assume his characters are normal. I typically play either repressed characters or genuinely sexless ones-- and this aspect of their personality is often a roleplaying hook. One player's characters are almost all high on libido, though it's generally expressed either romantically (as in, forming a relationship with an NPC) or being a charming, flirty fellow in the local tavern. These, too, often become plot hooks.
There's almost no inter-species sex in our games, except within the Humanoid type, or Outsiders. This is usually in a character's backstory (his heritage), though I doubt there'd be much problem if it came up in-game, though of course the usual fade-to-black would still occur. (Though, in the case of Evil Outsiders, we'd be sure to mention various scratches and bite-marks left behind. Hell, we do the same thing with Drow.) Chances are, any sort of interest in a distinctly non-humanoid species would be awkward, and mostly handled with teasing and jokes.