We already know obesity significantly lowers testosterone, and we have never been fatter.

Per the linked article, about midway through:

> Obesity can’t explain the entire decrease, however. One large-scale, recent review found a testosterone slump despite researchers’ adjusting for the subjects’ body mass index. “Even among normal-weight men, we see that same decline,” Eisenberg said. Some analyses, he added, have suggested that testosterone has dropped off most profoundly among normal-weight men.