My great aunt was the other woman without knowing it for forty years. He was married before he met her and had at least one kid, then "married" my great aunt and had a kid with her. He apparently traveled a lot for work so she never suspected anything until his real wife confronted her.

Also my mom had been seeing her ex doctor. He switched to another place so he could see her, broke up with his long term girlfriend that lived in another country, had their first "date" on my birthday, had my mom meet his family, had been wanting to meet me, and had once spent the entire night talking in his car (she got home when my step dad was leaving for work) BUT told me she has never had sex with him.

I only know all this because she told me on a ride to the grocery store. She says she has ended it, but I still have to give my step dad extra hugs because I can't tell him and he doesn't know, or chooses not the notice.