Let me give you an answer from a different perspective: not what Young Adults are reading, not what appears in modern YA fiction, but what I was reading as a teenager (12-16), and how it made me feel.
I was not reading YA fiction at all. I felt it was too simplistic, written down to some audience, not offering me enough food for thought. What I was reading were 18-19th century classics (Victor Hugo, Jane Austen), as well as 50s-70s fantasy and science fiction (Tolkien, Le Guin, Zelazny, Asimov, Clarke). In regards to your question, all this literature has a thing in common: while sex and violence happen, they are not explicit. Sex in particular, is more often alluded to than shown at all.
Reading the Iliad at 15, I was rather disturbed by the vivid descriptions of guts spilling onto the sand, and dying warriors screaming in pain. I didn't drop the Iliad because of this, it offered enough to compensate me, but I was disturbed.
My first encounter with explicit sex in literature was in Mists of Avalon, when I was 16. It was, moreover, sex from a female POV, thus closer to me. It was very much a shock. Not that I didn't know where babies come from, but it had been, up until then, something private, not really talked about. And as far as my personal experience went, I haven't so much as kissed a boy by then.
Before you think I was an exception in being sheltered, our history teacher in 9th grade showed us 1984 in class, and we all found the sex scene rather too much, detracting from our ability to discuss the more important ideas of the film.
So, to sum up, if your descriptions of violence are too explicit, too gory and shocking, you risk alienating your target audience, or at least distracting them from what you're trying to say. However, the issue is more with how explicit it is, then with what is actually happening.