I'll take the downvotes for you. Various very plausible drivers IMO:

- Plastics in EVERYTHING throw off hormone balances, including both metabolisms and... other things hormones affect. It's not just BPA.

- Dramatically different ages and rates of childbearing. Both lifestyle and hormones again.

- Pornography has completely changed how many men (and some women) interact with society. Lifestyle and... hormones.

- What percent of Americans are on some form of chemical antidepressant? A lot.

I'm sure there are dozens of other plausible drivers, and the answer isn't any single one.