Even if marriage was defined as a union between a man and a woman before, it would simply be inefficient and costly to create an institute entirely separate from marriage which ultimately serves the near exact same purpose. It's just a waste of tax payers' money. Besides, having gay people call their union something other than marriage (like "shmarriage", as has been humorously proposed) is pretty similar to the idea of "Separate But Equal", which was the public policy towards schools for white children and black children in the early 20th century USA. "Separate But Equal" was deemed an unethical policy by the supreme court back then, and I doubt results would be different for the marriage issue today. If they are to be truly equal, then there cannot be a separation.

And it's not like the definition of marriage hasn't changed before. It wasn't too long ago when marriage used to mean a union between a white man and a white woman. Then it got changed. Dictionaries aren't really prescriptive of definitions in the grand scheme of things, they are descriptive. Otherwise there wouldn't have to be new editions made every now and then. Definitions of words change all the time.

And I don't see how gay people getting married in anyway affects or attacks the marriage of straight couples. I can't see how it "corrupts the whole institution of marriage", as I've sometimes heard it said. That's like saying that when slavery was abolished and black people were given basic freedoms, it somehow lessened the value of the freedoms of white men. Or that when women were given the right to vote, it somehow deteriorated the voting rights of men. These arguments are nonsensical. Expanding basic rights to more people does in no way take away from or attack the people that already have said rights.