I have mixed feelings about the right to gay marriage, particularly as differentiated from civil unions. It's an important symbol of equal treatment, which non-marriage civil unions don't offer. Yet what is important to the couple is the rights and privileges the legal status brings, such as the ability to make medical decisions for each other, inherit, share pension benefits, etc. These are important, but are in some jurisdictions fully covered by civil unions. Beyond symbolism, what is important about marriage?
I am not concerned by the religious definition of marriage. I would never seek to gain religious sanction for my marriage, and if I did, I would only be interested in receiving it from a religion that wanted to do so. Religious entities certainly can define marriage as they see fit, as I can choose not to be part of one that doesn't support me.
So to explore the question of gay marriage, we need to consider the government interest in defining marriage. This is the entity that we lobby, that unifies the definition within a country, and that provides the tangible benefits to married people. Why do governments provide these benefits? What would be the government interest in extending those benefits?
Many gay marriage opponents argue that marriage exists to support procreation, and therefore should only be available to heterosexual couples. It might be that this was a main reason for developing this structure, historically. In the modern world, however, we suffer from too much procreation rather than too little, so we do not need government intervention to increase it. More proximately, if procreation is the defining reason, than marriage should be unavailable to heterosexual couples who do not--whether because they choose not to have children, or fail to. A married couple who does not have children within a reasonable time, say 5 years, should have their marriage annulled. Since this does not happen, it is clear that this is not a central purpose of marriage.
It has been argued that a major government interest in marriage is in supporting social stability. Couples support each other emotionally, which may have a significant impact on the average mental health of the population. Couples also help each other in life, for instance by sharing expenses, allowing one to go to school while the other works, etc. If one partner needs medical care or other high-cost support, the partner plays a significant role in taking care, reducing a huge potential drain on social services.
I think these are all good reasons to support relationship forming, but I don't think the institution of marriage is required to provide these benefits. Couples don't require government sanction to support each other. In some legal situations like medical decisions and joint expenses, formal recognition of the relationship may be important, but those don't require all the trappings of marriage. Another important consideration is that these social benefits do not inherently require only two people to take part in the contract. Polyamorous relationships, intentional communities, extended families, etc. can all provide these structures, often more effectively than a simple marriage. Not all people value these kinds of relationships, of course, but if the government interest in marriage were in supporting social stability, it should provide equivalent legal supports for these structures as well. It does not, so social stability also appears not to be the government interest in marriage.
Absent the above reasons, I find it difficult to imagine what other reasons there could be that governments sponsor marriage. The only other I can come up with is the perpetuation of tradition. Governments and legal systems arise in a historical context, created by people with a particular culture and goals. A significant role of government is (for better or for worse) to keep that culture stable. Laws reflect the values of the culture, and enforcement of the laws make it difficult for the culture to evolve in practice. As far as I can tell, the only interest the government has in supporting marriage at all is a momentum from a time when perhaps it made more sense.
The problem for gay marriage advocacy with this is that it takes us in the wrong direction. When marriage as currently structured made sense, gay people were persecuted, gender inequality was a defining aspect of marriage, oppressive values significantly limited individual achievement, and the economy had a completely different foundation. All those things have changed and continue to change more. Marriage seems to be a holdover from an older context without tangible value in the modern world.
This does not mean I oppose the institution of marriage. Many people still value it, and it makes them happy. While I see few benefits for governments to define marriage, I also see few harms--as long as the fairness issues of social benefits and responsibilities are worked out. But because I see little value to me in marriage, I question the value of advocating hard for the right to gay marriage. The social stability argument above seems to be a major reason in favour of the institution, but the gay marriage advocacy tries to enfranchise one group without meeting the full goal, e.g., by recognizing multi-partner relationships. Discussion of that would be less successful at this moment in history, but that is not a reason to advocate for a partial solution.
Beyond that, I think seeking the right to marriage takes us in the wrong direction. We seek to adopt a structure that has been a key part of our oppression, without consideration of the modern context. This is metaphorically seeking to take on the shackles. Instead, we should allow the institution of marriage to join the other aspects of our culture's difficult past. While I see no need to repeal hetersexual marriage, I think fewer and fewer people will see the need to avail themselves of it, particular as the surrounding rights are equalized. We should make heterosexual people's rights more like ours, rather than make ours more like theirs.