Denying same sex couples the right to marry isn't something that makes any sense to me. Given that, at least in my country--the United States--marriage is a legal contract made with the state, and has been declared a fundamental human right by the US Supreme Court the opposition to it is, somewhat, mind boggling.
It literally has no impact on how religious groups define marriage for themselves, such as a sacramental, covenantal union as it is in Christianity. We are, as a matter of civil law, talking about a state recognized social contract.
Even if you are morally opposed to how homosexual persons live their lives, they are still equal citizens under the law of a secular republic who are deserving of the same fair treatment under that law as all citizens are. As such they are entitled to the basic civil liberties all its citizens are entitled to. One of those civil liberties is marriage, that's not up for debate, that right was recognized by the Supreme Court decades ago when the US Supreme Court ruled that anti-miscegenation laws were unconstitutional--on the grounds that the right to marry is a fundamental right of man.
It is, as a point of fact, unconstitutional for the state to prohibit two consenting adults the right to marry; no less so than denying mixed-race couples the right to marry.
And no, this doesn't lead to a "slippery slope" toward men marrying toasters or their ostriches. The issue is, and always has been, about consenting adults; not toasters, ostriches or whatever else. But consenting adults protected by and beholden to the Constitution.
And note: None of this has to do with the morality of how homosexual persons live their lives--that topic is verboten and I understand that. I am instead speaking rather specifically on a matter of secular, civil law, specifically US law.
-CryptoLutheran