Havoc, you did do the right thing. The people that will say otherwise are fine with it -- until it's their wife... selective morality is a bitch, you know...
Almost anybody, put in the right circumstances, will cheat. You should feel good about your personal conviction not to become entangled in that mess. It might be good in the short run, but most of the time, affairs are found out -- and then it's a whole world of mess. It leads to destruction of marriages, dissolution of families, kids losing all sense of stability in their life, etc. Hell, some guys would kill you if they found out.
As the potential other man (OM), you should feel proud that you are turning it down, some wouldn't. It's a hallmark of stength, integrity, honor and self-confidence.
It's true, married women are almost as likely to cheat as married men. The difference is only about 5-10 percent between men and women adultery rates.
The thing is, maybe she will find somebody else to fuck, but maybe she won't. Either way, there is plenty of opportunity for you to pick up unattached pussy that you can have a relationship with (or not) and avoid all the drama that will come with screwing around with married women. She's obviously got issues -- the pussy might be good, but the person behind it obviously has problems, who knows how deep they run?