Why are some women more sexually active?

depends on how young. You said women. I'm guessing 18 and over. Most natural reason is: they want to reproduce. For some reason we are created or you could say, hardwired to want to reproduce. Hopefully, it will be a long thought out decision. Raising a child is a HUGE responsibility often without any support, and I am certainly including the father of the child. Grandparents could go either way.