Gay marriage, origin of marriage and other things?

Ok, first of all let me put this out there. Do not call me a troll. This is a serious debate that came up in my politics class that has been going on for over a week now. I am posting this question/ idea because I want to know what other people think about the opposing ideas and such. Answer logically, without bias, and without insulting one another. There's nothing worse than a person who can not voice their opinion without personally insulting another person! Anyway, In my class we spoke about religion and the government, we got on same sex marriages. A classmate argued that marriage is a religious institution or excuse for a monogamous relationship among heterosexual humans who are, of course sexual beings. He also said that aside from religion (Christianity being the main one, he said), the whole idea of marriage (man and woman legally united ect.) came from religion. Therefore, he said that homosexuals are actually infringing themselves upon marriage itself and not the other way around. (Doesn't make sense to me!) Another classmate brought up the separation of church and state. So, technically speaking, because homosexuality is viewed as a moral issue and not a government issue, he says that churches should not be forced to marry same sex couples? He said that the government is imposing itself on the church, because doesn't the original concept of marriage came from Christianity? Just what is marriage? Who came up with the whole concept of marriage? If it is true that marriage came from religion (Christianity, Catholicism, whatever ect) and is simply meant for one man and woman, wouldn't it be correct to say that homosexuals are actually forcing their agenda on something that wasn't meant to be for them in the first place? Please, don't get mad at me, but I've been seriously thinking about this! I'm confused and I don't know what exactly to think about it all...My teacher also says that it's not a human rights issue, he said that it is a religious and moral issue and in a sense the legalization of same sex marriage is not the governments business. So, I ask again, where did marriage originate from? Because it seems, that without "religion" marriage (one man and one woman or many women one man) would not be existent. Basically, what is being said is that homosexuals do not have the "right" to be married, because it was never ordained for same sex couples in the first place. Is this a logical argument? Does it make sense? I can see how it can, but I'm a bit skeptical!