Through the years I have had friends that have found themselves in this situation. I won't go into the judgement of it, but only the heartache. It never turns out well.
I've seen a friend marry one of these "Mr. Jones" only to find out he was seeing another "Mrs. Jones" some years later. She had been wife #3. He moved on.
Another friend had a 10 year relationship with a married man. 10 years. wow....Finally it ended. Probably like this.
From 1972, Billy Paul's "Me and Mrs. Jones".