In general, YA books have become fairly permissive. You can get away with a lot, if it works within the context of your story. The important question you need to ask here is: What purpose is the sex serving in your story?
If you don't know what it's there for, you'll have trouble writing the scene well. If you know it should have dirty bits in it but you don't know why, then readers will probably come out of it without much impression beyond "that was a dirty bit."
So the questions here are: what is your book about; what does the sex signify in the story; what kind of scene and experience are you trying to create for the reader. You can choose just how graphic you want to get, but the important thing is to know exactly what effect you're going for. If you're aiming for "sweet, tender first love," than using hardcore pornography term will not serve you well. If you're aiming for "naked, vulnerable, and utterly down-to-earth," then you won't want passionate, exaggerated euphemisms.
As a general rule, focus on feelings, not on mechanics. Feelings, character and relationships are usually what a YA story is about. You can reference body parts and intimate touches if you want to, but I think you'll usually find that getting into serious, explicit detail will be more of a distraction than anything else.