Interesting question. I for one have never been disgusted by thinking of my parents screwing. I don't have a problem if my daughter was in a legal healthy consenual relationship and if I walked in on her BF and her I do know what I would do.
As for joining in, sound fun in fantasy but IRL not sure if that wuld be something I would do.
I must admit I like seeing pictures of Moms and Daughters posing naked together and that I would definitely do.