[Mitsuru Nakata] Shiawase no uraniwa... | Behind the Happiness... (Kemono DIRECT 3) [English] [Mynock]
[中田ミツル] 幸せの裏には... (獣DIRECT 3) [英訳]
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It adds to the inmorality of the scene. Not only is she enjoying cheating, she's doing it with a guy that looks horrible and she doesn't care as long as she gets that pleasure. Though she didn't do anything with him in this one. Normally the girl in the stories is this pure (or at least nice) thing, so when going for netorare you wouldn't want a super handsome guy, you want the opposite corrupting her, a handsome guy wouldn't bring that feeling as much. And you want that bad feeling of knowing what you're seeing is wrong, and even so you are getting aroused (like the girl) by the scene of just pure raw sex and nothing more, throwing away love and relationships just for pleasure. This is what I think it makes netorare netorare. Sorry for the long comment.
Blessings of hentai