Sex and Doctor Who. For shame, people, for shame. Don't you have other people's childhoods to screw up?
I'm kidding, of course.
Sex is completely and utterly unimportant to the Doctor.
By unimportant, I don't mean he takes the act casually. Quite the opposite, in fact. Unless he can see a very, very good reason to have sex, he's more than willing to find something else to do instead. Universe is a big place, and he's got a time machine and a manic streak a mile wide.
I imagine the Doctor has had sex in the past. Depending on the canon you follow, he's had a grandchild, and therefore must've had a child at some point, so there's sexual possibilities there. Also, the notion of a man living 900 years and never having sex is pretty absurd, in my opinion. First of all, he's a man, not a monk. He's far from perfect, and sexuality is simply part of everyday life. The Doctor's not Bill O'Reilly or anything, he's a pretty attractive and fascinating guy, so he could get a lady if he wanted. He has, in fact, had several romantic entanglements throughout his time both on the show and implied for off-show, though nothing sexual has been, for lack of a better term, explicitly stated.
It is my opinion that during the "dancing" scene in 2.03, "The Girl in the Fireplace", the Doctor had sex with Madame de Pompadour, sealing his mental connection with her with a physical bond. I have roleplayed this interaction before with ambitious_woman, here and I've found the result to be one of the most...uh, again, for lack of a better term, satisfying NC-17 threads I have ever written. It fit canonically, and opened up many different avenues for roleplaying. It also didn't feel forced or wrong with my muse, furthering my belief that the Doctor can and does, on occasion, have sex.
The Doctor's relationships in my roleplaying with him occasionally do turn sexual, but I find that it is the storyline behind the sexuality that makes for much more interesting situations.
Sex for a Timelord, in the way that I've written him, is very similar to sex for a human, and is both gratifying and enjoyable. It's just not the most mind-boggling and amazing experience for the Doctor. He's also too easily distracted, I think. Can be rather frustrating.