How exactly do you propose that a change in socially acceptable behavior in the span of about 40 years had a tangible effect on hormone levels? It's not like someone can say "oh getting into fistfights isn't cool anymore, better reduce my testosterone levels". Maybe if we were talking about a timespan an order of magnitude or two longer then sexual selection (high testosterone and more violent men being less favored as mates) could come into play, but again for a time span of 40-50 years that's just not feasible. Also, socially acceptable or not, violent and aggressive men typically don't have problems finding sexual partners; Ted Bundy, the notorious serial killer and rapist, was known to receive many love-letters in prison, even after the details of his crimes (exclusively against women and girls) were made public and he was able to marry and conceive a child while incarcerated.