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.
While (assumedly) true, I feel the need to note that the reverse does not hold generally: 'man boobs' are purely fat in nearly all cases and will go away when someone loses weight.
Gynecomastia is breast tissue and does not go away if you lose weight.
https://www.cdc.gov/obesity/data/obesity-and-covid-19.html
I don't see a solution. We're not going to be able to use the same techniques used to get people not to smoke to address the obesity problem.