Serial Cheater

I have a question for women that stay with their spouse who is a serial cheater. How are you okay with your spouse cheating on you repeatedly? How do you work things out? I've seen several people say that cheating wouldn't end their marriage and they could work through it and it confuses me. I just don't get how someone could be okay with their spouse fucking another person  without them knowing and then they find out and just forgive them over and over again. I get maybe one time (although I have no tolerance for cheating and even once would end my marriage) but several times is ridiculous. I'm not bashing someone who is with a serial cheater I'm honestly curious. I mean you might as well as be in an open relationship if you're okay with them sleeping with other people.