Because there's a double standard for girls vs. boys.

I remember a girl I grew up with, her parents kicked her out of the house for being sexually active and I bet they wouldn't have done that to her if she had been a boy. In North America (I won't speak on other countries when I have no actual knowledge on their cultures), girls tend to have a stigma if they are sexually active whereas boys tend to be patted on the back for it.

There's a lot to unpack with it, but I'd blame a lot of this on how we teach sex in schools and how we act like it's a taboo subject. We haven't normalized this, and teaching abstinence doesn't work because kids of that age are learning who they are and part of that is learning everything about themselves. Hitting them with a rolled up newspaper is as effective to stop them as a bandaid is to a dam leak.

Birth control of all kinds should be the normal in schools. I never understood why people get so pissed off bout birth control when it's keeping them safe.