My OW met xh through a hobby and slept with him during a difficult stage in her marriage, when they both got drunk at a party.
They both agreed it would never happen again, was a mistake, but of course it did and then they agreed it was just a casual diversion. A bit of fun to escape their humdrum married lives.
She told me she never thought about me, it was as if I didn't exist. When forced to confront the fact that he was married, she convinced herself I was awful and he deserved better. She said it gave her a kick, knowing she could tempt a mm away from his family, it meant she must be prettier, funnier, better at sex than me.
Of course feelings developed and she started asking him to leave. She left her husband, which meant they could now meet at her house almost daily. For years he strung her along. She said she knew he didn't want to leave but she loved him so much, hated me, knew she could make him happier.
When I found out I threw him out. He moved in with her the same day. She sent me jubilant messages saying he'd chosen her and made the right choice. He spent years, literally years, asking to come home but she won't know that and I wouldn't have him back if he paid me.
A different outcome for me op, but I did get insight into ow's thinking because she asked to meet me and we talked for a long time. I think she was being honest.