Dear Imagrownup,
I asked FWS that question several times in several different ways and pretty much always got the same answer.
Because he was too much of a coward to leave.
Therapy helped uncover the reasons for his actions.
1. trying to "recapture" his first real love. Not literally as finding the girl and approaching her, but all his FB's had several of her physical attributes.
2. proving to himself that he was just as important/popular as other guys. He was only 5'6" and grew up with an inferiority complex and he compensated by having sex with other women.
Weirdly enough....his vision of himself if he had left was a middle aged man living with his mother having meaningless sex with whoever said yes. Pretty pathetic.
But his vision of himself before I busted him was a successful married man, two kids, nice house, nice wife, well respected but having a secret sex life to prove to himself that he was still a player and just as good as any of the other guys because he could have sex with anyone he wanted.
In my case, FWS did the hard and ugly work to fix and change himself. He had told me that the last few years had been the best of his life. He was well respected, had friends, and his life had purpose.
And I can verify all this because so many people attended his funeral that the Church was full and people couldn't get in. The Processional took nearly 30 minutes and the funeral itself made our local paper with a picture.
He touched many people and credited me with literally turning his life around.
He told me he was grateful that I didn't leave HIM. That I woke him up to what he had.
So you see, Iam...sometimes maybe they stay because they realize they need us to save them from themselves.
I've also asked myself why I stayed in the relationship. Maybe you can ask yourself and search for that answer. It might give you some perspective.
k9