To those who support gay marriage, the issue is an open and shut case of basic human rights and equality. From this perspective, it is insane and wrong to ask that GLBTQ citizens be treated as though their love, their lifestyle, their desires are inferior or sinful just to satisfy one loud contingent of the population. Allowing gay marriage would affirm the humanity and agency of all citizens, instead of just the ones who fit a certain, subjective idea of "normal."
To those who oppose it, the issue is much more complicated, regardless of what you believe as far as equality and human dignity. Most don't oppose gay marriage because they think GLBTQ citizens are inferior, or even primarily sinful people. Their opposition to GLBTQ lifestyles has to do at least in part with the fear that once gay marriage is allowed, those who continue to believe that gay marriage is a sin will be ostracized, forced to act against their beliefs, or persecuted for them; that they will be shamed and bullied and pressured by the mainstream.
It's easy for gay marriage advocates to shrug off this fear. "Of course that won't happen," they say. "No one will be forced to do anything." But part of the reason they're not worried about that happening is because if it did, they don't really care. They believe that those who think being gay is sinful are ignorant and hateful, and if those people don't get their way, good--because they're evil.
Flip that around, and you're back on the anti-gay marriage side of things: they don't believe that it's bad to ban gay marriage, not because they want to oppress or harm gay people, but because they sincerely believe that gay people aren't happy.
If you read my other posts it should be clear to you that I find it misguided to tell others what will and won't make them happy. But I think it's just as harmful to ignore the concerns and fears of those who disagree with you. We should no more shrug off the potential marginalization of Christians than they should shrug off the marginalization of the GLBTQ community. Each group thinks that the other's views are harmful to society, and therefore don't need to be addressed. Each group, in defending their own right to exist and speak up, is also diminishing the rights of the opposition to exist and speak up.
There is a possibility that once gay marriage is legal everywhere, it would eventually be illegal for anyone to deny it, no matter what. Everyone wants their way all the way, and they're willing to fight to the death because they're so sure they're right. The crux of this issue is that if gay marriage is legalized, some churches might be oppressed in their right to believe that it is a sin, but as long as it stays illegal, the GLBTQ community is oppressed in their right to believe that it isn't, and live as though it isn't.
So how about this: why don't we kick the government out of this debate altogether? We already have a legal certificate for marriage AND an optional church ceremony. What if we just called that legal certificate "domestic partnership," and the optional church ceremony "marriage"? That way, churches which don't believe in gay marriage don't have to marry gay couples, and churches that do believe in it can marry all the gay couples they want.
The government won't be dictating any kind of morality in either direction, except this: rights are rights, and all consenting adults deserve the same rights.
No church would be forced to acknowledge the marriage of/perform a marriage for anyone. On the same token, everyone's partnership is acknowledged/respected by every government body. Marriage remains intact, rights remain intact. Everyone wins.