Common things discusting to find out (Family Edition)

Which is most discusting to find out to the least discusting to find out, whether or not you have all the members of a kind of family:

Finding out your teenaged daughter is having sex with more than one guy,
finding out your parents are having sex with each other,
finding out your older brother is having sex with a minor,
finding out your lil sister is having sex with a man twice her age,
finding out your son is having sex with a hooker,
finding out your lil bother is having sex with himself,
finding out your older sister is having sex with another woman,
finding out you've been having sex with who turns out is a cousin of your's,
finding out you had sex with a transvestite, and
finding out your wife/husband is having sex with your best friend.

^^Put em in order from most to least discusting to find out in your opinion.

Why is sex so terrible anyway? Why is it everyone is concerned with who is sleeping with whom and how and under what circumstances?

It's hyped so much that people have actually killed each other over sex and lack of sex.