How far back does this millenials have less sex thing compare too? Today's women that grew up in the 90s yeah probably is more free then one from 50s and earlier. But compared to the 70s? I don't think it's farfetched to guess that society's feelings towards women sexuality have regressed since the sexual revolution of the 70s. Paris Hilton having a sex tape coming out was a huge deal, that's what millennial girls grew up with.

I don't mean to suggest that this is the only reason, I just think it contributes.