Sounds like it probably might happen. A group may keep a girl just because of that reason, but they're most likely not terrible. However I'm sure there are girls that use that to be carried, only due to the fact that it's a smaller population so they're more likely to be treated differently. I really can't think of a way guys can just use their gender to get in groups. I'm definitely not accusing you, however I think what you're describing is actually a "thing".