"A girl needs a reason; a guy needs a place."
I don't know the first person who said this (or a variant of it), but this is one of the truest things I've heard about female sexuality.
You once mentioned that both males and females had "two sex drives": the physical one, and the ego one. And I think much of the internal anti-slut defense can be attributed to the latter. They need to either have investment from the guy they're screwing, or feel like they weren't responsible for the sex happening: "it just happened", "I had to, I needed the job", "he lied to me, told me this and that." Why do you think rape fantasies are so common? Because rape is the ultimate "it wasn't my fault" when it comes to sex. They get to satisfy their physical drive without hurting the ego.
So I think it very much comes down to the ego sex drive. If they just put out without some investment or an excuse, they'll feel bad about it, cheated. So that's why it's the guy's job to make it feel like it happened naturally, and never really leave them an explicit say in how things progress to the sack. Chicks like to maintain deniability all the way - just look at all those MeToo harpies insisting that going with a man to his hotel room is in no way the beginning of a consent to sex.