Something that just came to me…
What is considered beautiful, admired and even cherished when in its proper place among others of its kind, but is considered unclean and revolting when found alone?
An internal organ
A hair?
A tooth?
A finger (if you live in the USA)
Genseric got it! A hair.
hee I like pravnik’s answer too, though 
Someone having sex? (I have a dirty mind.)
I was thinking something to do with sex.
Actually, the reason I thought of it was simple. I was slicing up some fresh pineapple (yum!) and I realized that a hair was on the cutting board, lying in the pineapple juice. It was my own hair. “Ew!” sex I to myself, as we are all likely to do. Then I stopped to think about it. I love my hair… but not when it is in my food or on my soap or in my drain. Only when it is on my head.
Freudian Slip?