C'mon - when I was a teenager I would've had sex with the crack of dawn if I knew it was interested. I doubt anyone capable of getting pregnant is truly 'off the market' for every teenaged boy in their vicinity.

It's more likely that popular representations of teen pregnancy in the media gradually taught enough kids that having a baby in their teens will seriously screw up their life. You only need to see one episode of Teen Mom to know that's nowhere you want to be.