My fiance dropped a bit of a bombshell on me last night...what would you think/do?

My fiance and I have been together for almost 2 and a half yrs (known each other longer). Before we were together, he had been honest with me about flirting/fooling around with his cousin's wife before we were together. No ...just fooling around in other ways. It took me a while to fully trust him after that...I have my own very strong opinions about affairs...anyways...I had always felt that he was keeping something else from me, and last night he finally told me something else that he and his cousin's wife had done. It hurt me that he would keep it from me...and something else tells me that there's something else yet (call it woman's intuition if you must). I guess that my concern is that if she comes onto him again, that he won't be able to resist. I can foresee that happening too...she is 30 years old and her husband of the same age is already developing symptoms of MS. What would your first instinct be? I love him very much and can't see myself being with anyone else.