Why do husbands never leave their wives?
Most men still love their wives, they are just feeling bored, neglected, or do it because they are acting out. Sex doesn't equal love and long term attachment forms a deeper bond than infatuation. If they are still attached and love their wives and you threaten their marriage and relationship (by telling the wife), they will likely lash out. I am speaking from my experience knowing a mistress whose house was broken into and whose life was threatened by the married guy she was hooking up with. This isn't true with all but it is an extreme example of a common reaction.