In essence - those females in society who can't get a man so pretend to like both sexes in order to look less pathetic and vice versa with males. More commonly female.
Note: Not to be confused with genuine bisexuals who enjoy both sexes and don't need to make themselves look less pathetic.
Girl 2: No, you just tell yourself and everyone else that because you can't get a man. Meaning you suffer from social bisexuality.