[candu]

Easy: a lot of states teach abstinence-only sex education, and there's a prevailing set of social norms (often imposed by "pro lifer Christians") that makes frank discussions of birth control, sex, reproductive biology, etc. all but impossible. It sounds crazy - and, IMHO, it is - but large parts of the US operate as de facto theocracies, especially where it comes to education.

Under those circumstances, it's not surprising that teenagers have sex, don't know how to do so safely, and end up pregnant without meaning to. That is: a teenager absolutely can choose to have sex without choosing to get pregnant, simply because the society around them has conspired to make sure they don't understand how one leads to the other.