It's simple memetic pressure. We've made out getting pregnant to be a bad thing. My wife and I married somewhat young, but hardly extraordinarily young -- age 22. When we conceived the first time, I remember going to the drug store with her to buy a pregnancy test and feeling ashamed at 23 to buy a pregnancy test -- ashamed that I got my wife pregnant! But it's simply due to the social conditioning that we're subjected to as teenagers. There's a weird American puritanism about sex, and it doesn't come solely from the religious right.