Many people against gay marriage say it should be between a man and a woman by definition. Some of them suggest a civil union that would have all the same legal rights as a marriage.
This suggestion sounds like a "Seperate but equal" policy this country had with African Americans 40 years ago. The problem is that it didn't work then, so why should it work now?
The very idea of marriage isn't logical from a strictly legal/financial standpoint. It exposes the individuals in a couple to extra liabilities and few extra benefits from a strictly factual viewpoint. So many people do it for the less tangible benefits. They do it to show the world who their partner is and proclaim a binding commitment to them. A civil union simply can't have that same impact since it doesn't have the same history or emotional attachment that the concept of marriage does.
I could start listing reasons on why marriage isn't technically logical or why seperate but equal doesn't work, but I would probably just be belaboring the obvious here. I figure some people will debate these points and I'll take such debates as they come.
The point is that "Seperate but equal" in practice was never quite equal. Why should it be suddenly now?