That doesn't sound good. Flirting is one thing but cheating well that's something totally different and not something I'm sure i could cope with. Whatever path a couple take it should be mutually agreed beforehand in my opinion. Whatever the reason she had for doing it, she should have tried talking to you about it if it hadn't been discussed before. Waiting two years to come clean and only after you had admitted to flirting, which seems to me the perfect time to tell you as she can throw back at you aren't entirely innocent, isn't a good starting point for this lifestyle. Honesty and trust should come first.