The first question out of my mouth was indeed "Why??" She said it was because she would feel more protective of a daughter and more inclined to try to keep her innocent.
She has a very Victorian outlook in some ways (I think that's the right time period) - men are supposed to be men of the world, experienced, etc, and women are supposed to be innocent and chaste.
For the record, we were engaged long before we moved in together. I'm no slut.