Cheating in the marital bed

I've heard that even when having an affair, bringing the other person into the home, or the bed you share with your spouse is rare. I think that most of the time when people cheat they are already checking out of the relationship, but I can't help but wonder about the significance of going this far. Probably has a lot to do wanting to get caught, or wanting to hurt the person you're cheating on, or wanting the affair partner to replace your spouse. In any event, I consider this to be the ultimate F-You to the marriage and your spouse, and something I could never get over or forgive.

Why do you think a person would do this? Is this the ultimate betrayal when it comes to cheating?