Exactly the reason why you don't grasp why she, as beautiful woman who could have every partner, would go for an old man and his ugly son is why it works. A handsome man is just an alternative version of the MC. When she decides to have sex with people who, under normal circumstances, would be out of her league though, the MC (just like you) will have difficulties projecting himself into her sex partners which either alienates her or increases her depravity (from his point view). When appearances become irrelevant, the sex tends to become more primal.