>>81520131 (OP)As far as I can tell, there are two reasons:
1. Women have higher standards for casual sex than they do for relationships. So it stands to reason, that the kind of men they're having casual sex with they *generally* see as much more attractive than their eventual partners. This creates an environment where sex can be seen as more fun and less like an obligation, opening up the possibility for experimentation.
2. Casual sex partners are less committal. Even if a woman has a fuck buddy that she has sex with regularly, it's easier to break things off, so there might be less pressure to perform a certain way during sex. If she experiments with a long-term partner, there might be an expectation to keep it up, even if she's not happy about it, which could create tension.