I disagree.
First, talk to her; then seek counselling.
Both of you.
If all else fails, get a divorce. I am not in favor of it, as a Christian, but it's better than having side chicks. Why?
Marriage Vow is a sacred covenant. Christian or no Christian, you've made a promise, sir (whoever is contemplating on breaking it). Does it mean nothing to you, so you want to cheat on your wife?
If all else fails, get a divorce, and find someone else.