Well, it's not only sexist and involves ridiculous gender based double standards, it's also completely homophobic (this may seem too obvious to even bring up).
But, this assumes that the all the girls will grow up with the desire to date/ marry men. And, obviously the whole gay marriage issue throws a wrench into the "wait until marriage" idea for gay girls. (Again this seems too obvious to even bring up).
I can imagine scenarios like the "purity ball" culture probably force lots of teenage lesbian girls deep into some kind of closet where they end up going along with conventions and behaviors that have nothing to do with their own desires or personality (and only have anything to do with parental expectations).
And, what if a straight girl grows up and wants to remain independent? Why force anyone into marriage? Things like this make it seem like an artificial institution forced on kids by their parents (society, etc.), at least with in "purity ball" society.