I have to admit that in writing for the YA market I am not sure where the cut off is between YA and adult. I am generally of the mindset that it falls somewhere between 7th graders and high-school seniors. Let's face it, kids today have been exposed to so much worldly supposedly "grown-up" stuff from TV, the movies, print and electronic media and YouTube and who-knows-what when they surf the web. Many of them think that they know about "everything" when, in reality, so many of them are so emotionally immature they probably do not know what reality is or is not, especially when it comes to the world of sex.
I feel that writing for the YA market is not just about story telling it is also about being responsible to your youthful audience. There should be a sense of the finer human traits we possess like caring, camaraderie, loyalty, honor, and selflessness woven throughout the narrative. It does not have to be blatant, just subtle, enough for the reader to sense it and be impacted by it. You never want to be "preachy". Sexual interactions between a boy and girl should be approached carefully and gently and never be explicit. The reader will get the idea without all the pronounced description. We are human beings after all with feelings, emotions and the ability to reason. I think it is important to show we are not just creatures primarily controlled by our desires but rather that we, as people, have the right to say "No" or "Stop" and in return hear back, "Okay" or "Yes" and that this type of behavior is honorable and deserves respect.
Let me finish up by saying that the YA level of writing should probably also be an enjoyable read for adults. I have found some great reads at the YA level and I would recommend that adults who have not tested the waters of this genre ought to "jump in."