Judging by your post it does not sound like you feel bad about it cos you betray your GF's trust. You just don't want to get caught. So since you do not have a moral issue with it might as well go full 9 yards. P.S. How do you know that your GF doesn't know? Maybe her mother told her and they both are trolling you and laughing behind your back? Or maybe your GF is secretly hoping that you will make a move so she can jump in because she is very aroused about the idea of threesome with her mom involved? Or maybe... ...maybe you are immoral sinner and need Jesus!