They've been impregnating the most feminine women they could find. If generation after generation mothersare busty hourglass thicc etc etc, would you act surprised if boys produced from such unions over (presumably) centuries of such artificial selection become more womanly as well?
That literally makes no sense in a genetic sense, men want the most feminine woman because they are the most healthy and likely to have plenty of children, women like masculine men because their genes are stronger AND can protect their families.
If boys come from masculine men, they will adopt their most masculine traits from their fathers, not their mothers, of course, it depends on how the boy was raised, how the pregnancy was handled, was the mother taking care of her hormonal balance?
To me it sounds like an allegory to our modern times, this is literally happening whit our generation, this generation of men is the most feminized that is has ever existed, but why? many MANY reasons, but the easiest one is, bad food, lack of exercise, and lack of good healthy routines to keep your T levels high, but if you ask me why in game this happened, my first theory is that the Goddess grew bored of the Masculine heroes, and wanted to feminize them slowly to see how society would change, my OTHER theory, is that the Goddess had nothing to do whit it, but instead these old Heroes never actually made sure to leave a legacy of HOW they were so masculine and succesful, but the WEAKER ones did, see how the small dick hero leaved a myth for every hero onward, while the Masculine ones are almost forgotten since they were a LONG time ago, and they HAD to be succesful otherwise they wouldnt be Heroes, i think the original heroes set the standards, but each generation slowly crippled away the foundation (also remember that Erin basically villanized Old Heroes thinking they were brutes and old fashioned, and that idea had to come from other people, so they have been villanizing Old heroes for being Hyper Masculine and driven by their desires while succesful) and the priestess groomed each hero to be a submissive provider instead of a traiblazer that did whatever he felt it was necessary to do.
Basically, you live, you get trained, you fight the Demon king, you win, you get the wife, you reproduce, you die.