Elaine was fucking someone behind the MC's back the whole time, this is pretty much the definition of NTR so... If that doesn't excite you then that's also why it didn't excite the people who complained
I've now replayed now it's been rewritten it and it's quite funny, she tries to pass off her sluttiness as just being a nympho and therefore not slutty. As others said I'm not quite sure why the dev needed to add that in, it's also a really weird conversation, someone attacks her and instead of just explaining why that happened she goes off on an unrelated tangent about how she's obsessed with sex. Lovely to know love.
Also there's a really unpleasant scene where MC creampies her, then she sits on his face without asking... That's just... I mean cmon, ask first.