You are wrong. Infidelity does not nullify the marriage legally, or in the eyes of God. Period. You have no basis for that argument.
It gives standing to the wronged party in a divorce filing. Are you actually arguing that Terri would ALLOW her husband to father children with another woman? The only reason divorce papers were not filed is because the one person who is authorized by the court to file is the person who is committing open and notorious adultery. Bit of a conflict of interest there, yes?