Let me begin by saying I'm not gay and I'm not married and neither of those things is likely to change any time in the immediate future. That means I don't really have any stake in the gay marriage argument. That said, here are my two cents on the subject.
On the one hand, gay and lesbian couples want the same legal rights and protections as heterosexual couples. I can't say that I blame them. As far as I can tell, from my completely unscientific and rather limited observation, most people don't have a problem with that.
No, the problem seems to come in when religion gets involved. Most of the arguments that I've heard against allowing gay marriages stem from people's religious beliefs. Given that it's no surprise that one of the biggest sources of support for California's Proposition 8 was the Mormon church. Which I find rather ironic given that Mormon marriage practices are considered rather questionable themselves by pretty much every religion that doesn't practice polygamy. Just goes to show that being discriminated against doesn't preclude you from discriminating against others.
Now I'm not one to believe the Constitution requires total separation of Church and State. The men who stamped "In God We Trust" on all of our currency weren't afraid of religion. What they were afraid of was a church effectively taking control of the government with church officials governing the country instead of our duly elected representatives. This is what the First Amendment says:
Congress shall make no law respecting an establishment of religion, or prohibiting the free exercise thereof; or abridging the freedom of speech, or of the press; or the right of the people peaceably to assemble, and to petition the government for a redress of grievances.
That's a command to not establish a state religion and to not interfere with people's religious practices. Now obviously we do interfere in some religious practices. To use an extreme example, if you practice a religion that calls for human sacrifice you're going to be out of luck here in the United States. However, gay marriage is not human sacrifice.
There are a lot of religious practices that offend me. For example, I find it offensive that Catholics believe that it's okay to sin as long as you confess your sin to your priest and then do whatever penance they give you. (Yes, I know that's not the Catholic church's official position but it's how pretty much every practicing Catholic I know views things and it's a rather obvious mindset given the church's rules.) It doesn't matter if I find that offensive because the First Amendment gives them the right to believe whatever they want.
What I find far more offensive than gay marriage and isn't sanctioned by the First Amendment is using the Constitution to make religious discrimination the law of the land. This is exactly what people supporting a Constitutional Amendment defining marriage as being between a man and a woman want to do.
Your church doesn't approve of gays getting married? Then I guess gays won't be getting married in your church. If there's a church that believes it's perfectly okay for gays to get married, and obviously there are, then there's no reason they shouldn't be able to get married in that church and you can console yourself with the thought that they don't have a proper marriage just like you undoubtedly tell yourself that any religious practice you disagree with isn't a proper religious practice.
The government isn't supposed to support one religion over another but that's exactly what's happening here. The obvious solution is for that since marriage is clearly a religious institution the government should get out of the business of defining what is and what isn't a proper marriage. Instead let each religion decide for itself what constitutes a proper marriage.
Of course if the government isn't going to be sanctioning marriages then it shouldn't be attaching legal rights and protections to them either. Now it is in our society's best interest to confer those rights and protections so let's confer them through government sanctioned and completely non-religious civil unions with existing marriages being grandfathered in. Then people can get married in whatever church they want and just file a notice of civil union with the government. As an added bonus people who don't want to have a religious ceremony, such as atheists, but do want something a little more elaborate than standing in front of a Justice of the Peace could have some sort of civil union ceremony. It might even create a small industry around civil union ceremonies.
Seems to me like everyone would win except, perhaps, those people who can't stand the thought that not everyone shares their religious beliefs.