What I think is that you're snowballing and trying to divert the topic so you don't have to answer the question. I notice that Puck has already bailed on the topic so I'm guessing no one is going to answer.
I don't have the answer you're looking for, but I will address the question.
First, I know most people believe the only people against SSM are against it for religious reasons... there was a poll years ago that showed that to not be true. I can't find it now but it asked people their views and then asked their religion, and it was really surprising that a lot of people who didn't identify as Christian were still against it. I don't think you would find that true NOW, because the numbers have started to switch with more people pro and fewer people against, so I would guess we are probably getting closer and closer to religion being the dividing line, but I'm not sure because it doesn't look like anybody has repeated that study. Or at least my Google Fu isn't finding it if they have. =P
OTOH there are gays who don't think gays should be allowed to marry. Gore Vidal is probably the most popular and vocal one, he thinks marriage as an institution is already destroyed and doesn't understand why anybody would want to take part in it. (
it's strange to read his comments on it.) He's been with his partner for a long time, too, I don't remember for how long but it's been AGES. You would think "of course someone like this would be for gay marriage!" And yet he isn't. Hence why I said it's strange to read his comments on it... also that he seems to be calling it fighting for your right to get divorced... ehhh. Makes me feel uncomfortable in a way I can't describe.
So there ARE arguments that aren't "GOD SAID NO" against gay marriage, some even being made by homosexuals themselves. I honestly don't know what a lot of them are, it doesn't exactly matter to me, so I can't tell you what they are. But I do know they exist.
Now, as to whether any given individual thinks they are "good" arguments or not... well, that's all subjective, isn't it? I'm sure to Gore Vidal, his arguments are good, but probably aren't good to a number of other people (like I'm sure Elton John thinks he's full of crap!)
Does that make sense? Sorry, I know this probably reads as "devil's advocate" stuff, but since I'm not a non-religious person against gay marriage, I just know they exist, I can't really argue it from that perspective or tell you why they feel that way. I think it probably boils down to either what Gore Vidal has to say about how marriage is crap (though why that changes it from a rights issue I'm not sure; there are plenty of straight people who don't want to get married either because they also feel marriage is crap...) or they are just very traditional people. Like Puck said, even though our country was founded under the belief that the government could not FORCE anyone to follow any particular religion, nor punish them for doing so, a lot of our founders were religious and built our country on their beliefs. So it makes sense that "traditional" in our country would be pretty in line with religious beliefs... so even if somebody doesn't identify as religious, their beliefs could still easily be in line with religious beliefs...
To me I just sort of like to boil things down to "be excellent to each other," but I'm a child of the 80's. =P