There is a serious debate, in this country and in Europe, about whether incest should remain illegal as it has from time immemorial. When the Supreme Court declared all state laws against sodomy an unconstitutional intrusion on a citizen's right to have consensual sex in the Lawrence case, alarmists claimed that bigamy, polygamy, incest, and bestiality would follow. The alarmists were right.
I have written about the coming polygamy crisis. Incest is next.
Of course, nothing is inevitable in social life. We could have a new social movement to restore a public standard of sexual morality. But the movement to make sex, even marriage, a matter of private choice is well advanced. The Supreme Court has pretty nearly dropped the last legal defense. It would be possible to amend the constitution to prohibit incest, but it would be horrible. Liberals would feel obliged to go way out on a libertarian limb, from which it would be difficult to come back. My reading of the present political moment is that the movement toward social conservatism has run out of steam for this generation, and is not likely to be renewed soon.
One of my favorite college professors caused a wonderful scene during a polite dinner with some visiting evaluators. He grew increasingly frustrated as he tried to get them to draw any practical judgments from their philosophy. Finally he rose, and in a carrying voice, demanded to know if they could offer some reason why he shouldn't have sex with sheep - though he put it a bit more graphically than that. Not, mind you, that he wished to do any such thing; rather, he wanted them to explain if their philosophy had any conception of what was good, of how people should live.
We are not quite to the "sex with sheep" stage in the debate over the complete privatization of sex, but we are only a step away.