Assume that the social landscape remains the same as now in western countries, with both men and women allowed to be promiscuous and without any economic pressure to marry.

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My personal answer is that it won't happen, I don't have a scientific proof but it seems to me that eventually most unattractive men manage to reproduce during their lifetime. Furthermore, as we know the bar for women to have sex is low and most women are probably going to reproduce anyway regardless of their beauty, thus (statistically) not all of their sons are going to be good looking.