There is a young woman who was in love with another woman. They were a couple for a couple years. Then one day the main girl finds her lover in the arms of a man. It took a while for her to forgive her lover. Then one day the main girl is getting ready to have some kinky sex with her girlfriend. She is tied up and gagged. The next thing she knows is the man who had been having sex with her girlfriend is now having sex with her. After the main girl is untied she moves away. A few years later they run into one another and the ex is surprised to find out the main girl has a twin children. The ex asks if she was so against having sex with a guy and she hated everything to do with it why not abort or at least put the baby up for adoption. The main girl says she had thought of both options but her parents had forbidden her from doing either.
Another idea could be that there was no betrayal at all. The main girl was going to be moving and she hand known that her girlfriend was bisexual and had been letting her have sex with a guy all along. And before she left the two girls set it up to have the main girl have sex the guy her lover was doing it with and the betrayal was just sex play. Maybe the guy knew about the acting and maybe he didn't. I still say the girl got pregnant. And once they got reunited they get married.