Are promiscuous girls more insecure?

I have a couple of friends who are really nice ladies but seem to be dating a new guy every week. we bump in to their friends and they say "oh, i dated him by the way." often the boys are totally out of limits...for example, a thirteen year old hooking up with an eighteen year old. do you think these kind of girls are just insecure and hate being alone, or are they sluts through and through?