That's the best reason you can come up with for restricting the civil liberties of an entire group of law-abiding, tax-paying Americans?
What's happening in the UK can't honestly be traced to gay marriage, especially since full-fledged gay marriage is NOT EVEN LEGAL there.
Legal recognition of same sex relationships has been going on for 20 years. If anything, polygamy and whatever else you want to dream up would already be legal in the nations that have had protections for gay couples the longest, such as Denmark, which started providing civil unions in 1989. There has been no "domino effect" which has granted legal recognition for polygamous marriages there.
But your previous argument was about "tradition vs. contract". Well, TRADITIONALLY, polygamy has been much more common and accepted than same sex marriage, so why do you oppose it? In fact, the polygamy issue in the UK has its roots in traditional Islamic ideas of marriage. Is marriage about tradition or not? Make up your mind.

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