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You just can't disconnect sex and morality, although any two people may disagree over where to draw the line.
I do agree with this to some extent. For example, one of the first few respondents mentioned that her boyfriend once had sex with a homeless person in an alley. That could have some elements of him exploiting that person's bad situation, and that has moral implications about the type of person he is/was.
A threesome, however, with consenting adults is less problematic. I agree that the LR's boyfriend was being unreasonable to focus on this. For one thing, he can't unring that bell. And for another, the act in itself wasn't wrong.
As far as Chasing Amy goes, for me the big issue is that the girlfriend lied. Not about the scope of her experience, but about being this committed lesbian for whom Holden was her first male lover. There was some attractive draw to her being this lesbian that he was able to "win," and she even admitted that she knew that it would make their bond seem more special. There was an element of surprise -- not only that she was very sexually experienced, but that she wasn't the previously committed lesbian that she had portrayed herself to be. He put her on a pedestal, but she helped as well. And that's wasn't cool.