Hypocrite! First get rid of the log in your own eye; then you will see well enough to deal with the speck in your friend’s eye.*
-Matthew 7:5, NIV
If your cheater does not understand and affirm that the presence of the Other Man/Other Woman is a problem, then your marriage is doomed without any hope of a rebuild.
You have a threesome–or more–and not a godly marriage, folks!
Now, I highly doubt the cheater actually believes what he or she is saying when they claim ignorance over how a sexual or otherwise inappropriate relationship outside the marriage is a problem for said marriage. They didn’t believe it at one point or why the secrecy?!
Pastors and Christian leaders, this sort of response to infidelity exposure is a sure sign the cheater is unrepentant. You cannot repent of something that you are still claiming isn’t a problem–i.e. is an “innocent” relationship.
Plus, such a response demonstrates complete, callous disregard for their spouse’s feelings. It is ice cold and contemptuous.
Plus, they have just spoken in a way that indicates that they have no problem violating the Ten Commandments with impunity. It is the sort of response I would expect from someone who has absolutely no fear of God. So, treat the cheater accordingly.
The faithful spouse is not safe in such a relationship and expect further harm from someone as that person has just told you their priority is to continue harming the marriage plus their spouse as they see no problem doing so in the past.
Personally, I would recommend pulling the faithful spouse aside and telling them as much. They need to know the truth even if it is a hard truth. That is true kindness and wise counsel.
Plus, faithful spouses need to know that the mercy of divorce is offered by God for situations just as this one (see Jeremiah 3:8, Matthew 19:9, I Corinthians 7:15, etc.).
A faithful spouse does not need to stick around in such a marriage where the cheater has declared his or her priority is to serve himself to the further harm of the faithful spouse (and family). If they want to damn their soul to hell by continuing in adulterous sin (see Hebrews 10:26-27), they don’t get to continue harming their spouse on their way down. The faithful spouse is free to walk away from such a marriage.
Cheaters are free to give God the proverbial finger, but the faithful spouse does not have to follow their example. They can divorce such a godless person without shame.
* I quote this verse as cheaters claiming to see no problem from their end often go on to talk about all the ways the faithful spouse is a “deficient” spouse. All this takes place while the log of adultery remains protruding from their own eye.