There's very strong evidence that both sperm counts and testosterone levels are going down (wikipedia it) worldwide over the last 60 years.

The mechanism here would be that many plastics are or contain endocrine disruptors. Or just an unknown mechanism (there's just so much about biology we don't know).

I believe there's also evidence that younger generations are having less sex. There's also a claim that people have much less "mature" (i.e. sexually-developed) facial features.

Of course this could all be tested by a proper study.

Now back to your point, your first reaction might be "All my friends aren't having kids because they don't want to, none are trying and can't." But if sex-drive is down across the board that may explain it. America doesn't even self-replace (unless you count new immigrants).