I think it would depend on how much the modern human had to drink that night, and were any of his friends around to see the Neanderthal girl in the morning. That's how it played out for us when we were young- You could get with a chick that had a unibrow big enough to tie to your pickup truck as a bumper, you just don't want your friends to know-
I don't think we've changed all that much in twenty thousand years, do you?

Birth control as well.