I think it comes down to 'purity' and loyalty of relationship. I bet not everyone finds pleasant to bang a main woman (i.e. mom, sister etc.) whose being sexually exploited or close to it by other men even if it's her own husband or boyfriend, though it creates a sense of competition for main character desperately attempting to gain lover interest from opposing threat. Of course, this varies from person to person so my two cents in the end, but if game's main theme is 'mom and son sexual discoveries' then it should focus on that with, of course, optional side storyline to randomize game experience a bit.