Rod Dreher asks the key question: "If God doesn't exist (that is, if there is no such thing as absolute moral truth), why shouldn't the woman have sex with her brother?" This in response to an article from the Times Online about a British academic who had "consenting sex" with her brother since they were teenagers and saw nothing wrong with it.
There are certain human activities that there is no strictly rationally reason to condemn, but have been universally abhorred by civilized people. One of them is incest. Another is cannibalism. The only objection you can have against them is a religious one. Yet there are still people who claim who reject religious explanations for things who want to condemn them.
But it won't last long.