To sex or not to sex; It's a heavy question

For those who write in an adult genre, especially Romance. Sex is a bit of a non-issue. Readers expect your characters to have sex early and often. But for those of us treading the murky waters of YA, this is a huge decision.

after the first kiss
What happens after the first kiss?
I think most of us will agree that there is a certain level of non-disclosure that happens if a sex scene makes it into a YA novel. It's a good rule to leave out the details and focus on the strong emotions surrounding the moment.

But before we get to that point, we have to figure out if our characters have sex in the first place.

I'm working on book two in my Watcher trilogy right now and I'm starting to ask myself these questions. In book one, my characters' relationship was new and they were still trying to figure out how to be together. But now, they are growing closer with every hurdle they overcome. And I'm now faced with a tough decision.

In terms of my character, I think she's ready (or as ready as any teenage girl can really be). So the question becomes is my story ready for it? Am I ready to add in another layer of complexity? And if they take this step now, am I writing myself into a hole for book three?

I'd like to get your take on this, especially from those of you who are as crazy as me and choose to write YA. How do you know it's the right time for your characters to make the leap?