There is something I have been thinking of, long and hard. SM didn’t just run across a man to have an affair with. She went to FB, to find a man living in another state, her “almost” ex from HS, with a trove of kids.
Her behavior doesn’t look like a serial cheater to me. Rather, it indicates that she was sick to the brim of BM and was ready to use any make believe, any “I invented him” person to escape from the reality of her life. Beautiful women who produce a strong impression on men and know of it won’t need to look for someone from their past; there are tons of men around. Between two states, IN and CO and BM working all day long, it is not difficult. If you impulsively go to a person from your distant past it means this is the last person you considered seriously, or thought, loved you.
I strongly suspect, JMO, that SM viewed her having two kids after cancer a miracle and somehow connected this miracle with marriage to BM. He was boorish and gruff, but he gave her kids and probably she thought, he loved her. Her second round of illness - again, only JMO - was an eye-opener about his true (lack of) feelings about her. Her looking for the relationship shows such desperation to me. Poor SM.