[cyncooper]

(Adult)Brother/sister, father/adult daughter? Polygamy?

How 'bout those as better analogies for adults legally capable of entering into a contract but are banned by law.

[cyncooper]

banned by law of getting married, that is.

[Lunatic Fringe]

Those are better analogies...

But one can argue against them that genetic defects as a result of that type of relationship would threaten the quality of life in American society. You cannot make that argument with gay marriages.