I oppose gay marriage because I also oppose regular marriage, at least the state recognized kind. Which is to say I don't actively oppose gay marriage at all, though I don't lift a finger to help as well because I'm certain that:

• Victory on gay marriage is assured.
• I am quite confident the next issue the cultural progressives will focus on will be a destructive one.

Imagine an unstoppable mad man. He spends one day tearing down a damn, which results in flooded villages. The next day demolishing a wall in a hospital that causes some patients to die of exposure. The day after that sabotaging a power plant causing economic damage and chaos. If you see him carry on like this day after day for many months, and then observe him one day piling off the prison bars where an innocent man is being held, would you as a presumably good consequentialist really want to help him? You are sure he will manage to do it, but it will take him all day, if you help him he'll just go do the next thing that strikes his fancy.

Now obviously if you consider the mad man a morality oracle, a prophet of sorts, you have to help him! But what if you see him more as a force of nature or amoral process much like evolution? But I digress.

Marriage is a personal or religious arrangement, it is only the states business as far as it is also a legally enforceable contract. It is fundamentally unfair that people agree to a set of legal terms and cultural expectations that ideally are aimed to last a lifetime yet the state messes with the contract beyond recognition in just a few decades without their consent.

Consider a couple marrying in 1930s or 1940s that died or divorced in the 1980s. Did they even end their marriage in the same institution they started in? Consider how divorce laws and practice had changed. Ridiculous. People should have the right to sign an explicit, customisable contract governing their rights and duties as well as terms of dissolution in it. Beyond that the state should have no say, also such contracts should supersede any legislation the state has on child custody, though perhaps some limits on what exactly they can agree on would be in order.

Such a contract has no good reason to be limited to just describing traditional marriage or even having that much to do with sex or even raising children, it can and should be used to help people formalize platonic and non-sexual relationships as well. It should also be used for various kinds of non-traditional (for Western civ) marriage like polygamy or other kinds of polyamours arrangements and naturally homosexual unions.