The latest xkcd is an odd one. I know some people freak out a teeny tiny bit at the thought, but it never bothers me.
I'm a first child, and I calculated back when I was conceived, and estimate that it was almost exactly the day of my parents' wedding — which was an elopement. The two of them ran off at a young age to Idaho where they didn't need to get parental permission to marry, and right away they had me. I find that wonderfully romantic and have always had the knowledge that my parents loved each other very much (and were also a bit crazy and impetuous and careless…well, and also loved kids a lot). The squeamishness about parental sex has always seemed a bit weird to me — don't people want their parents to be happy?
Of course, I don't want to know the details, OK? That's personal and private and should remain between the participants, no voyeurs allowed.
And I definitely don't want to know about my kids' sex life. I just want them to have a happy one, and that's enough knowledge for me.
Hmmm…maybe that's the root of the fastidiousness—a concern for the privacy of the individuals involved?