An Affirmative Defense
If you file for divorce on grounds of adultery, your spouse is entitled to raise something called an affirmative defense if you forgave him for his behavior. He doesn't have to deny that he did anything wrong; instead, he can argue to the court that you should not be able to use his wrongdoing as grounds. Your forgiveness gives him an affirmative defense called condonation. You can't revoke your forgiveness, or take it back.
Condonation involves the logical assumption that your forgiveness is conditional upon your spouse not committing the same wrongdoing twice. If he commits adultery a second time after you forgave him, you can file on adultery grounds – provided you don't forgive him again.
You don't have to worry about issues of condonation if you file for divorce on no-fault grounds. All states now offer this option, so it's not necessary to cast blame in order for the court to grant you a divorce. In some states, such as New York, you can cite irreconcilable differences. In others, such as Maryland, you can file after a 12-month separation. In either case, your spouse can’t file an affirmative defense to no-fault divorce.