I'm not going to tell a one-night stand that I have a girlfriend
If you have a girlfriend then why would have you have a one-night stand?
I'm fed up with girls. The last one I was with told her friends that I'm gay.
I'm sorry, but cheating is never forgivable. You should just break up with your girlfriend if you're tired of girls. You don't need to cause any more harm to your her, as the cheating will devastate her.
I have never been gay and am not gay. I just do not understand why a girl would do this.
I believe you when you say you're not gay, but if you're fed up with girls then that's not a good thing for your girlfriend to have to deal with. Add that to the cheating then you are doing her a big disservice by staying in the relationship. It would probably be best for both of you if you part ways.