(picky point but true) Gay marriage has always been legal. There's no legal prohibition against gay people getting married.
(what they really mean) Same-sex marriage, on the other hand, does not exist, and it never has. Marriage in every civilized culture throughout history has been between one man and one woman, or between one man and multiple women, but the multiple wives were not married to each other.
So the issue isn't "should we let people of the same sex get married," but, "should we redefine the institution of marriage to include same-sex couples." Every time the issue goes to vote, it's rejected by a convincing majority, even in "blue" states. So clearly it's not what people want to do, either now or any time in the past.
I believe you could pretty well strike the word 'civilized' if you provide that 'cultures' must span a reasonable number of generations. Marriage is probably the most universal of all cultural features.