DOMA and Prop 8. Both History.

[Hammerskjold]

Isabella I married her cousin Ferdinand II. If I'm not mistaken their family is still reigning in the Netherlands.

Never should have happened but it's easy to get confused about these things:

http://www.youtube.com/watch?v=RW4vOpOEGhg

Then there's the case of Springfield V Shelbyville:
(from http://www.imdb.com/title/tt0757018/quotes )

Jebediah Springfield: People, our search is over! On this site we shall build a new town where we can worship freely, govern justly, and grow vast fields of hemp for making rope and blankets.

Shelbyville Manhattan: Yes! And marry our cousins.

Jebediah Springfield: I was- wha... what are you talking about, Shelbyville? Why would we want to marry our cousins?

Shelbyville Manhattan: Because they're so attractive. I... I thought that was the whole point of this journey.

Jebediah Springfield: Absolutely not!

Shelbyville Manhattan: I tell you, I won't live in a town that robs men of the right to marry their cousins!