The problem here is that it sets an unreasonable burden.
1) You normally have no way to prove it.
2) Some women are too shy to come out and say it.
It in effect lets a woman declare consensual sex to be rape if she changes her mind. (Or a man, but that's normally not an issue.)
If a woman is too shy to say it, but not shy enough to not want sex, perhaps that should serve as a warning sign for a woman who might regret her actions.
Just don't have sex with this kind of woman, guys.
Tell her to come back when she's not too shy to ask for what she wants.
Easy peasey.
It's a cultural issue.