...And afterwards, she feels guilty and violated and horrible about herself, even though it was her decision.

And, afterwards, Bob feels a lot less attracted to Sally, for reasons he can't explain, and after a few months starts chasing someone else.

Genes influence us by emotions, and our genes and our memes often have goals that conflict. A lot of men appear to have something like a madonna-whore dichotomy: pursue women sexually, but dump women who let you have sex with them quickly, and marry the ones that hold out. This makes great sense from his genes' point of view: a child you father but don't have to raise is a pretty high benefit at low cost, and the expense of raising a child is so high he should make sure that the child is his (and sexually reserved women are less likely to cheat).

There seems to be a conflict between the strategy she consciously expects to work- have sex with whoever you want with only health consequences- and the strategy she unconsciously expects to work- sexual reserve is a finite bargaining resource, and she just misspent some of hers.

Should she discount the sexual reserve argument because that's not her milieu, and no one will think worse of her for being sexually available? (How would she feel if she overheard Bob crowing about their encounter to mutual friends and acquaintances? What would she think about it?)

Should she discount the conscious strategy because it's costly (if it's possible) to overcome emotions? (Was the difference between Bob and a vibrator worth feeling terrible afterwards, and overcoming feeling terrible? If she knows having sex early on dramatically decreases her chances of a long-term relationship with Bob for emotional reasons, should she trust he won't feel those emotions / can overcome them through thought?)