I am in the dump-them camp too. If you are not married kicking him or her out of the house is the only sane option in my mind. If someone could not be loyal to you before you are even married chances are they probably wont after you married. If you are married get hold of a divorce lawyer, before you do anything. Especially if you have kids and house and stuff. I have seen a few cases where things got really messy because people acted brashly. Whatever you do, dont hit him or her - divorce courts don't look too favorably on violence. My humble opinion. Take my advice at your own risk :)