I think it has to do with a lack of self-acceptance and frustrated sexuality, more than anything else. I can see someone with poor self-esteem fantasizing about having sex, especially after a long period of abstinence.
Also, the female is mistreated as a slut, or depicted in suffering. While showing a lack of emotionality and vulnerability in the male, which often lacks expression or even a face. This might have to do with some form of emotional abuse or shame, experienced by the male, now being projected into the female.
Generally, the mentality and psychological age of female characters does not correlate with the physical appearance of their bodies. It's like having woman whose mentality behaves more like a child, inside a mature body. Perhaps this games are meant to play into the lack of understanding of sexual relationships between adults, present in dysfunctional people.
I was asked to take a look into this kind of genre, by someone who was curious about this kind of material.
Also, try Ariadne The Incarnation. Amazing hentai game with the classic "innocent" baby-face looking heroine with the body of a pornstar, having sex with ugly men and other creatures. It's a great rpg on its own, and can be completed without playing the adult content.