> I'd guess it's a combination of this and the overweight/obesity rate climbing

I agree, but for a different reason. Excess fat is an endocrine disruptor: adipose tissue makes Aromatase [1], which converts testosterone -> estrogen.

[1] https://en.wikipedia.org/wiki/Aromatase

  Adipose tissue is the greatest peripheral 
  source of aromatase in both males and 
  females,[citation needed] contributing 
  to the production of estradiol.

- https://en.wikipedia.org/wiki/Adipose_tissue#Physiology (middle

Normal Testosterone levels -> normal libido.

Excess estrogen causes problems for both male & female bodies.

I wonder if obesity is the cause for the multi-generational decline of testosterone levels.

It's probably a factor that can't be denied, just like reduction in fitness levels (e.g. exercise), less physical labor, masculinity shaming, etc.