Why are so many girls becoming promiscuous?

It's a little something they call being liberated as a woman. Breaking from the chains of chauvinist is what they'd say, or something similar. I'm a little worried about what my future kids would be influenced to do now. Also, it makes it a little hard for me to find a mate, considering the fact that many girls who choose to be promiscuous decide to, for some reason, lie to the guy they're with and say they're not promiscuous, then after a few weeks or so, she breaks his heart. I don't understand why. What purpose does that serve?