One thing I have noticed in this debate is that generally the people who oppose gay marriage seem to feel that the "gay lifestyle" is a choice. While I would agree there are gays who act extremely irresponsible, you can say the same about heterosexuals. I personally feel being gay is a strictly biological thing, in most cases, and not something that people choose to do.
Just for a second, think about this. Let's say you are a "normal" heterosexual male, and obviosly, are attracted to women. Do you think you could suddenly change the sex you are attracted to? Do you actually think you could decide one day that you prefer men sexually? IMO, something like attraction and sexual preference are purely biological. When you are attracted to someone, it is something that happens totally naturally without thinking. You have a physical reaction like raised heart rate among other things. These involuntary responses are not something that can just be switched to the opposite sex by choice...it just makes no sense.
Now, that being said, if you are currently against gay marriage/civil unions, and you feel being gay is a choice, not a biological "condition", and it was proven to be something that a person is born with, would you still object? If so, why?
Also, one more point. If being gay was indeed a choice, why would anyone choose it? What is there to look forward to? Being shunned by society? Having to hide your sexual orientation for fear of backlash or violence from homophobes? Not having several basic rights that "normal" couples have? Where is the upside that is making these people "choose" to be gay?