@Mrs Whiggins nails a lot of it. It’s very similar to obsession with purity in super-rigorous Muslim cultures.
Purity codes developed for a time a place when STDs killed, and illegitimate children exploded social bonds and communities. That’s also why girls were married away at about the time today’s Americans hold these “purity balls.” That’s why dads took it seriously, he would literally lose money if his daughter was “impure.”
I actually agree with a lot of what @BeardIP says. There are lots of reasons that a high view of human sexuality would encourage sex in committed monogamous relationships at an age where you are grown up to handle it. It’s good for your interpersonal relationships to not be addicted to porn. I think that’s equally true for boys so I don’t get why you frame it as especially true for girls aside from the old adage that boys can’t become pregnant.
Besides being totally creepy, purity culture is misogynist as hell and incidentally, if you’re into that sort of thing— very much opposed to the gospels and epistles where physical purity rituals are constantly dismissed as legalistic and at odds with a high view of grace. Not that sexual ethics were discarded, but the idea of ritual impurity for any person was. I mean, he kind of says this explicitly when he forgives a woman accused of it and then elsewhere goes onto say that any man who looks at a woman with lust (something he realizes everyone does and will do) has broken the law just the same.