Incest is covered by the impossible consent issue.

What about between adult siblings? Come on, man. You know your brother's hot.

I would, however, say that there is a compelling government interest in restricting the rights granted by the government to go along with marriage to one person per person at any given time.

Seriously. Not that I'd have anything against a harem per se (though someone might), but think of the tax code! Gah.