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If you need any more convincing, try this thought exercise on for size. It's just another instance of trying to deny a marriage you wouldn't seek to other people who believe something different than you.
"Traditional" marriage is defined as convenient - in traditional marriages a brother could take his sister-in-law. Fathers were allowed to barter their daughters into marriage. Kings were allowed to deflower other men's brides. And yes, races were to be left "unadulterated" and "pure".
Tradition has rightfully been revised and corrected many times before, and to make the claim that allowing other people besides yourself to get married to people other than yourself is an affront to your free expression is brazen, to say the least.