Frankly, I don't think SCG really thinks that most people are going to be sexually attracted to children.
This seems to be more of a way to make (their idea of) modest dress a habit before the girls grow up, as well as a response to the oversexualization of female clothing at younger and younger ages.
But the way they push religion into it is just so annoying.
And why just girls? I mean, it's worse for them, but it's still sexist to just state standards for them.
The Myth Of Men Not Being Hot probably plays into this. There's a hilariously flawed idea that men are the only people who want sex, and women just sort of use sex appeal to get the babies that they really want instead. So men are not attractive, and women are, because only men experience sexual attraction.
It's bullshit, but it's a surprisingly common subconscious idea.