B PhD has a nice piece on "open marriage."
Best line: If you have slept with n people, then anyone who has slept with n+1 people is a slut. Our own experiences form the outer boundary of what is morally acceptable.
Amazingly perceptive remark. And it transfers to other areas. We are all immigrants, in the U.S. Even the "native Americans came here just 8 or 10 thousand years ago. But somehow, the arrival of one's own particular ancestors seems to have perfected the U.S. Before my people came: America sucked. After they came: America was perfect, and all additional immigration should be outlawed.
The KPC view--on marriage, let people do what they want. Rules=tyranny.
On immigration--let people live where they want. More people=better country.