It's not the only cause but, from my experience, it's a big one, no pun intended.

I might have made different diet choices as a young man if I knew that body fat aromatizes testosterone into estrogen. This is the primary cause for most cases of gynecomastia, which is why it's rare to see a man with breasts who isn't also overweight to some extent. Obesity is effectively feminizing a large portion of men and lowering their sexual market value. Overall, I think obesity is making people less attracted to each other, which can partially explain the decline in the rate of reproduction.