I think a fair number of "boys" get "girls" pregnant accidentally. Even a lot of "planned" teenaged pregnancies have an air of "oh wow, I didn't think it would actually happen" about them, you know? Like they were toying with the idea of making a baby, even to the point of having unprotected sex... and then when it happens, reality hits home and all they want to do is jump in the time machine.
But I agree with diamond in that marriage is not a solution. Maybe back in the day a boy was expected to marry the girl he impregnated, but with divorce rates as high as they are, what good would it really do? Even if they got married before they got pregnant, there's at least a 50/50 chance that they won't be together ten years from now. Having children won't change that.
I think people are too quick to make life-changing decisions. Making babies with people they haven't known long, rushing into marriage because they found "the one"... these are decisions that require a lot of time and patience. Most people aren't willing to wait that long because they think they have their life figured out. And they might, who am I to say they don't? But the statistics on couples staying together are against us all, unfortunately.