Does he have kids with his wife? Why are they dating other people? I would not have too much of a problem with it if they were all adults but I question your daughter.

It is obviously more than sex if she is bringing him home to meet you. So they are not just swingers. The man basically has a wife who is first and a girlfriend who is second. How strange is that. She is not a girlfriend. She is a mistress.

Most women should want to be first in their man's heart. That is what marriage is. The point is your daughter is second. Only a mistress. She could do better.