My late paternal uncle(we have the same last name), divorced his wife in the early 70's. They had two daughers together. His ex-wife kept her married name. She had a son by another dude and give him OUR family name. I guess she wanted to be consistent. Would you be okay if your ex-wife kept your last name, had a kid with another dude and gave that kid your surname insead of it's father's name? I have no idea whether my uncle even knew or cared. Me and my little sister kind of joke about this and think,"Dafuq?"