This is from James Taranto's column today at the Wall Street Journal online.
If you really believe that all sexual activity between consenting adults should be legally permitted, you ought to object to laws against incest (when no child is involved) and polygamy too. (The adultery example doesn't really fit, since adultery involves a usually nonconsenting third party, the betrayed spouse.)
Well, Jim, yeah, that is exactly what I believe. As far as I'm concerned, if everybody involved is eighteen and is acting on his and / or her own free will, what's the problem? Remember the bit about "the pursuit of happiness"? If my neighbor wants to get it on with his grandma and a billy goat on roller skates with a gallon of Crisco, that's only my business if they make so much noise they keep me awake at night. And don't give me any animal rights crap about the billy goat.