I think it never explained the infertility problem because it wanted the viewer to come up with their own conclusion. I could be pollution, the stuff that we eat these days (hormones and chemicals), or it could just be plain and simple evolution. Maybe we were supposed to stop breading and die out.
Yeah, none of those really worked for me. Maybe big chunks of the population would be affected by any one of those things, but not everyone on the freakin
planet.
Maybe I'm just too much of a science nerd, but that premise was just too much of a whopper not to have a better explanation. It would be like if everyone in the future had suddenly learned how to
fly, and the movie didn't bother to explain it. We'd all be going "Huh? Just how exactly did THIS happen?!?"