...because I'm lazy, I'll just paste the response I gave to this topic from another thread.
I guess you can only really understand the severity of that question when you're experiencing or have experienced anything like that, it's no good saying "oh I'd stick by him forever regardless of what he did" when you're unaware of the emotional instability this could actually cause you, it can actually be worse than physical pain.
I've been through my share of heartbreak and while I've done some really irrational things in the past, if I were thrust into that sort of situation again, I'd try to talk it out with him first...of course it would be a bit of a thin line because he would have basically broken the code of our entire relationship, so even if we tried to compromise...the hurt would still linger and the seed of doubt and suspicion had already been planted the moment you found out. So it wouldn't be unusual if that could entirely break a relationship eventually. The love may linger there, and while it's possible to start all over, trying to maintain a relationship after that would be easier said than done.