safety in numbers?

I think that you are right with the safety in numbers. It seems that there is still a stigma when a female goes out by herself. Being single myself, I am choosy of where I go without anybody with me. Thankfully, my height works to an advantage. It is different when a man goes out by himself. Or maybe when 2 females go out togther, one tries to outdo the other--some competition?
 
Perhaps they encourage each other with innuendo? Or perhaps one of them wants to show the other how sexy she is and can get a man? Who knows? The reasons could be as varied as all the women who do act sexier among their friends.