The reason that so much of this (purity rings, purity balls, etc.) is aimed at girls is very simple. Boys can't get pregnant. The equivalent function for boys would be to have a group of fathers who have daughters take the boys out to a working ranch on the day they're gelding sheep, horses, etc. And stand there, sagely nodding at the animal husbandry activities.
I've always been deeply grateful that we never had a daughter. I'm afraid that I'd have been tempted to lock her in her room as soon as she started getting girl-shaped, and not let her out until she turned 30. Originally it would have been until she turned 40, but Mrs. Tex said that was excessive. That said, the notion of marrying God is disturbing to me, because I always understood that was what Catholic nuns did -- they even wore wedding rings. I won't even go into "Daddy is my boyfriend". That's just sick if the girl is more than about six years old.