Hard question that one...
I have always advised people that it is really an individual decision, and if that decision encompasses forgiveness, then you must leave the past in the past, and not allow that past issue constantly impede your present. If you do, the relationship falls down regardless, except all you do is punish the person... its like a form of payback I guess, at the emotional level.
You simply have to make a true and honest decision. Can you ever trust the person again or not. The answer is that simple, regardless of all the financial or family aspects that surround a relationship. Love is a wonderful thing, but with love comes trust, and if that trust is removed and not truly forgiven, then all you do is prolong the inevitable vs. really repair the trust aspect of love itself.
Some people find cheating less of an issue than others... so its individual.
I agree with others, in my individual case, and my wife's, we both see it as the ultimate betrayal of our relationship. If you're that desperate for sex, then masturbate... as both sexes can do that.