What a living can't give unmarried gay partners is access to each others Social Security, insurance from work, and etc.
SS, no, but the SS program shouldn't exist in the first place, so that's not a point that matters. You should be providing for your own retirement and
that you absolutely
can give to anyone you want, married or no.
As for insurance, marriage doesn't guarantee that either. Your spouse can still be denied, or an outrageous premium applied, despite your being married. Your spouse having a pre-existing condition being a prime example. Your marriage license doesn't have any magic pixy dust to force the insurance company to cover most pre-existing conditions. The only exception I can think of is pregnancy, or if there's a carry-over clause honored with a prior policy. Usually COBRA will facilitate carry-over clauses.
I don't see any non-religious reason to deny gay people the right to marry. Especially when you consider the 1st Amendment, looks like a slam dunk to me.
I would only deny a gay couple for the exact same reason I would deny every other couple: belonging to a high-risk-of-divorce demographic. If your coupling belongs to one of these demographics, that can be resolved with pre-marital counseling, then you're not in a high-risk demographic anymore.
That's the way I see it, and I've been happily (Most of the time) married to my female wife for about 20 years. I see no reason to deny anyone the same rights that I have. Of course the fact that I'm a Secular Humanist has a lot to do with my attitude, I don't have some guy in the sky telling me what I should or shouldn't do.
The US Supreme Court considers Secular Humanism a religion, fyi, because it's an established philosophical outlook with common basic rules shared by all Secular Humanists. It's a non-deistic religion, like Buddhism. In that way, your view here is a religious one, so walk softly when condemning religious beliefs.
I don't relate to people on either side who give religious views any weight on the topic of legal marriage. Religion is separate from the law and as such cannot be part of our considerations of what the law should be. If your relationship is not otherwise harmful, then you should be able to attain a marriage license from the State, because marriage is about commerce and the state needs a compelling reason to deny your right to engage in commerce. I think having a high likelihood of failing is a compelling reason to deny you any kind of license. Even with a small business license you can only operate in the red for a couple years before the government pulls the plug; and if you demonstrate that you are likely to fail again, you may not be issued another license until you improve your situation.