What does "tradiational" marriage mean? Yeah, sure, boiled down to its absolutes, the traditionalists will come up with one-man-one-woman. But as soon as you take a step beyond that … well, even in the US, there's a lot of room for different interpretations.
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First-Cousin Marriage Much More Available Than Same-Sex Marriage
Conservatives use the threat of a slippery slope to incest as an argument against same-sex marriage, but they never address the fact the first cousins can actually marry in more than half of the states. The New Civil Rights Movement has produced a handy little Venn Diagram to show the comparison. It’s a stunning reminder that the definition of “marriage” is as inconsistent as the arguments used by those who claim to “defend” it:
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While first-cousin marriage is a handy talking point, it isn't a particularly severe genetic hazard if only carried on for one generation. As main populations urbanize and become more mobile, even less so. Even (!) sibling offspring are no more likely to have genetic defects than, say, a woman in her forties having a child to a man in his fifties.
It does address the moral principle of "I think it's icky, therefore it is wrong", however.
As well as, "Well, it's a long-standing tradition for marriage and marriage law, so clearly we should allow/prevent it."
I think there are thrice as many ideas of what “traditional” marriage is in the US as there are versions of the “Ten Commandments”.
@Marina – Echoes of Potter Stewart’s definition of obscenity, ” I know it when I see it.” (http://wist.info/stewart-potter/6862/)