Thanks for your response.
I understand where the expression comes from, and I agree with you that it is effectively a relic of the past. However, that logic is still often applied today. Have you ever heard a woman say “I’m not that kind of girl”, or a man say “she’s wife material”, or, especially, a woman say that she’s “saving the best part of herself” (i.e. refraining from sex in a relationship) until some sort of arbitrary goal has been met and the man has sufficiently proven himself? Women and men still buy in to this, and it’s only hurting everybody.
Also, you don’t really reference gender in your explanation of things today (when you’re talking about milk being easily obtained, there being little benefit to a legal relationship, and marriage/birth rates etc.), whereas the expression clearly does: it’s about a man — “he”—obtaining sex from a woman. I absolutely think that all genders should freely have sex and marry/not as they see fit.