Let's take a good friend of mine. Good marriage. Good kids. Then he goes bonkers at mid-life. Repents. She forgives. They have a wonderful marriage, and both love each other. I love sitting in their home. I can feel the love in that home, and it's not a touchy-feely sort of love. It's a love that's endured a horrific trial. One where one partner grieved, and the other partner had to face the fact that he was the cause of so much grief. Tell me, where is she better off? Living with her husband who now loves her more than ever, or living in an apartment with few prospects for remarriage, while hubby drives his little red sports car with wife number two? If you can truly forgive, and not become bitter, it's a no-brainer. If you're bitter, it's best to move out. And, yes, women have affairs as well.
Remaining in a marriage and seeking to heal *is* the better avenue...but many times affairs continue.
Often, when one partner has an affair, he or she will have others. The inclination to have affairs will not go away. The partner who has affairs has shown that he or she cannot resist the illicit love affair.
The innocent partner may rationalize acceptance of the love affairs from the standpoint of it is no big thing or not big enough to make me move out and face the prospect of being alone. The innocent partner may decide to also have affairs. But the damage is done to the relationship.
The partners may be able to work things out. Many cannot. It depends on the people themselves.