It's wrong to break up somebody's marriage
Katie is having an affair with Dan who is married to Jill.
Katie wants to be in a relationship.
Katie realizes that there are children involved, breaks up with Dan and starts dating a single man.
Katie tells Dan to leave his wife for her.
Katie's relationship with her new boyfriend hurts no one, and they become engaged.
Dan divorces his wife and his children grow up in a broken home.