Disclaimer: the following is an argument of logic, not religion.
THe only way your point is valid is if all religions are equally correct. But by definition they can't be. Assuming for the sake of argument the existence of a higher being (because without one, there wouldn't be a true religion), only one religion can be correct. By their nature they are all mutually exclusive.
Ergo, the argument that some allow it and some don't is inadequate to justify it's acceptability. It can only be justified according to the one true religion, whichever one that is.
So it's illogical to justify marriage between two people of the same sex based on religious acceptability unless you have evidence that the religion that accepts it is the one true religion.
On this point, I'm afraid, your argument is crap.
On the others, no such discussion is allowed. And even if it were, the two sides never start with the same premise anyway. Whether it's an issue of semantics or something much deeper, there is never going to be a meeting of the minds on defining marriage.
Gays went wrong with trying to co-opt the language. They'd have had a much better chance simply arguing that state-sanctioned unions disguised as marriages were nothing more than a contractual agreements between two parties and as such there was no legal basis to deny them the right to enter into such contracts with people of the same sex.
Crap argument back at ya.
The same argument can be used against marriage of a man and a woman then.
There are thousands of religions, im sure you can find one that supports only gay marriage and who is against the marriage between a man and a woman.
And like you said, there is no way to know which religion is the "true one".
So what you said can work the other way around as well.
So it's illogical to justify marriage between two people of different sex based on religious acceptability unless you have evidence that the religion that accepts it is the one true religion.
And like you said we cannot know which one is that "true religion".
So maybe gay marriage is the only "true marriage", who knows.