Why not? Because it's a crappy thing to do.
It just doesn't seem very Christ-like to play a role in someone destroying their marriage. It's not even a Christian thing - it's a Golden Rule kind of situation. I wouldn't want someone to be okay with sleeping with my SO, so I wouldn't do that to someone else. Cheating is something that doesn't have to happen. You can break up with an SO and/or end your marriage in ways that don't hurt your partner so deeply.
Of course, people have moment of weakness and make mistakes. I'm certainly not perfect. That's why it's healthy to stop and reflect on your values and life choices every once in awhile. Check yourself before you wreck yourself.