Personally, I think ignoring the fact teenagers have sex and refusing to acknowledge it in the books written for them is silly. It doesn't have to be graphic, or even 'on-screen', but teenagers are horny little creatures, and trying to write realistic characters without sex is difficult.
I'm not saying every character in every book needs to be doing it on a regular basis, but avoiding a sex scene where one is organic to the plot seems dishonest. There are sex scenes of various types in all three of my YA books, and the book I'm about to start has one too (this is going to be probably the most controversial one of all since the MC is only 14).
Writing sex scenes is hard. Especially in YA. You don't want to get graphic about it the way some adult romance novels do. It has to be simple and tasteful, but still show the power of the act. In one of my books the only sex scene is an attempted rape, and obviously that is portrayed in a very different way than the shy, fumbling attempt at expressing first love in another book.
How do you feel about sex in YA? Do you write it? Do you avoid it?