First, suppose that there was no such thing as state-sanctioned marriage. No tax benefits for being married, no deductions, no implications for social security credits. Instead, marriage would simply be something that people do privately to announce their commitment to their friends or their church or their family or their God. There would be no legal implications for this, only psychological and emotional ones.
Taking the government out of our private lives would have implications for family structure, too. There wouldn't be tax deductions for having kids, for example. Why should the government give people a financial incentive to have a particular family structure?)
Instead of making sure that your employer gives you leave if you have a child (biological, adopted, whatever), the government could make sure that everyone got a certain amount of leave time to do whatever they wanted. If you want to have a kid, great. If you want to write a novel or volunteer at the local animal shelter with that time instead, great.
It's not that people with families would be "punished;" it's simply that family-related activities wouldn't be privileged over other activities. Similarly, the Family Medical Leave Act (FMLA) could still exist, but it wouldn't just be to take care of a family member. Instead, you could use it if you needed to take care of anyone who was sick, even a friend.
I can imagine downsides to this approach, not to mention logistical difficulties associated with a lack of default rules about various matters (e.g., who can visit you in the hospital). Health insurance could be problematic, too (though, uh, if we gave everyone health care, this wouldn't be an issue...). But there's no reason we couldn't find solutions to these problems.
Since, statistically speaking, most people benefit from the laws and policies and practices that endorse particular family structures (and particular activities related to the creation and maintenance of these structures), I doubt that the government is likely to disentangle itself from these anytime soon. But when we talk about whether gay marriage is worth fighting for, I can't help but wonder if these fights are beyond the point. As long as marriage remains a government creature, I will remain fully dedicated to marriage equality. But maybe the real problem is that the government rewards and incentivizes particular ways of living over other ways, calling the structures it endorses "American values," and implicitly branding all others deviant. If this is so, it is a problem that goes well beyond gay and straight.
I'll be interested to know what you think about all of this, dear readers. Should marriage be a government creature at all? At the very least, I think it's worth pondering.