Do you believe in the phrase "Once a cheater, always a cheater"?

My girlfriend cheated on me with a guy that she used to have sex with a lot. The last time she had sex with him was in 2010 but he's been staying at their house for a little while...and somehow it happened. The guy told me that she started to cry in the middle of it, and they quit after about ten minutes. I can tell she's genuinely sorry; she cried in my arms all day, and I was so shocked and all I could do was just tell her that I still love her the same and that everything was okay. She kept saying that it wasn't okay and asked me why I wasn't pissed off. I just held her and tried to calm her. She really is sorry and she swears on Bob Marley (her hero) that she'll never do it again. I believe her, but at the same time I think I could be being naive. So, do you guys think there's a chance that she won't do it again? Or do you think that once you're a cheater, you're always a cheater. Btw were both girls, and we've been together since January. I really love her and I think everyone deserves a second chance. But yeahh, I want a second opinion.