[NSFW] Why do we find it so revolting to think of our parents having sex?

Recently I have had a few conversations regarding incest. I am trying not to “mix” up the different concepts here, but we were discussing siblings being reunited after being separated, then becoming sexually attracted to each other. The friend I was chatting to said of course, to imagine having sex with her siblings or her father, or any family member disgusted her. This is what got me on the following train of thought, and forms the question.

Then we chatted about how horrible it was to even think of our parents having sex, or being sexual. Or in fact any member of our families being sexual in anyway. Why do we find that disgusting? It is normal of course for ones parents to engage in sex. Whatever flavor they prefer. It is an act of love for the most part? Maybe some of us don’t, perhaps we find the idea OK? Would love to hear your thoughts on the subject.