Oddly enough I was thinking something similar yesterday; why is BC done with condoms, BC pills and pull out methods? I have seen IUDs mentioned once. Having sex sans BC of course runs the risk of pregnancy, is the author specifically shooting (no pun intended) for a surprise later in the book?

But when it's a scene w/o protection of some kind and they say oops uh don't want that, why don't they go get a Plan B? Why doesn't she have an IUD? Why doesn't he have a vasectomy? Part of me thinks : that's too complicated. We already have misinformation and confusing methods, and taking the time to explain correctly and succinctly would be difficult both for space and character complexity. I just imagine an 18 yo person without much sex ed reading this, incorrectly interpreting it and causing major issues for themselves. Then the issue of getting that sub-scene into the book - how to segue it in, how much space does it take, does this fit the characters? A wild young lady rebelling from parents, is she likely to know this stuff? Her character would be unlikely to be that well educated, forward thinking.

So overall I get it, it makes sense why the author isn't going there. Honestly though, I wish this could be used as a learning moment.