In trying to fit their round polyamorous selves into a monogamous square hole, they will ultimately fail and repeat a pattern of guilt and self loathing which contributes further to lower their self esteem. Their obvious failures and misery are compounded by the disgust of their peers, parents and often even their friends.
Casual sex should be with somebody you actually like. My new rule of thumb is to only shag men I don't mind sleeping over, and am okay with kissing goodbye when they leave. Anyone else isn't worth my time- and if I'm not prepared to make at least that much effort, I'm probably not worth theirs, either.