How much sex is too much?
Question: If I never go into specifics...how much description is too much for my book to stay young adult, as far as sexual encounters go?
These days, there don't seem to be hard rules in this area. Sex is certainly not required in young adult books. On the other hand, it's far from forbidden.
Of course, within some romance genres there are lines that have strict rules. Some "sweet romance" or Christian romance lines have no sex outside of marriage. That's just the publisher's rule - but it applies to adult books as well as YA. On the other hand, there is the Harlequin Blaze line, which I'm sure some YA readers dip into.
It also matters if you want to write a book that can be taught in schools. In some regions, sex in books can still get them banned from classrooms
Apart from such considerations, I think what matters is that sexuality is not gratuitous, that is there because it is an integral part of the story and the main character's journey. And it should be true to the character. If you're writing about a 15-year-old character's first experience, don't glamourize it. Don't suddenly give them a level of confidence and maturity that is out of step with their personality.
Also, it helps to have a sense of your ideal reader. YA readers range from 13 to 25 these days. My feeling is that, the less mature your reader, the fewer graphic details you should include (partly to avoid turning it into pornography, and partly because that's not what some younger readers want in a story). More emotionally mature readers can appreciate the details of sexuality in the context of the character's growth as a rounded human being. They want to read about characters like them who see and experience sexuality from a similar perspective. Some young people still wait for marriage, but most do not. YA literature should reflect this.
Let me refer you to an article that does a good job discussing this topic in reference to some YA novels which are intended for the older YA crowd...
Also, I would suggest you read a number of recent YA books to get a feel for the genre.