This actually happened to a family we know – Catholic family.
The “mistress” or whatever you want to call her kept the child, who visited the dad and the rest of his family every other weekend. He was several years younger than the rest of their children (I think they had 5-6 of their own, as I recall). The wife tried her hardest to be gracious. I don’t know how she really felt (my daughter was friends with hers, that was the connection). By the time we met them, they appear to have mended their marriage, the best they could, and were just making the best of it, the little boy was about 6. One day, one weekend, one birthday party at a time.
The mistress had to contend with having her little boy not get to live with his dad, while the rest of his siblings were raised in an intact family. I can’t imagine that was easy, either. Whatever she got in child support, the intact family appeared better off than her raising the little boy by herself.
From the outside, they seemed to file it under “not an ideal situation, difficult, let’s all do the best we can and move on”.