Just to clarify:
What I did not mean was that individuals are beneficial to society depending on who they choose to have relationships with. To me that's a personal choice the government has no place interfering with except when one person in the relationship is a child, etc, which we can all agree upon. Should people have gay relationships? I don't think there will ever be a common consensus, and I think that's a debate for the religious or the philosophical, not for legal statuses. In my opinion, who people choose to have relationships with is none of my business, and God knows I'm a man with enough flaws that I'm not in a position to try to judge other peoples lives. Certainly I'm not in a position to try to judge them based on their personal relationships.
In my mind there's a line between what behavior the government should regulate, and under what circumstances the government should grant institutions special legal rights. As I understand it, the government grants special legal status and protections when those institutions benefit the interests of the state. For example:
Corporations get special limited liability because allowing less risk means increased economic growth for the nation;
Giving small businesses tax breaks means increased small business growth which encourages entrepreneurship and innovation and consequently is good for the nation;
Giving the press more freedom with libel and defamation laws concerning public officials encourages honest critique of government officials with less fear or reprisal (although on a side note it's sad what passes for journalism often these days);
Making Senators and Representatives privileged from arrest "in all cases except Treason, Felony and Breach of the Peace" is granting them special status to prevent them from being detained so they can't vote which is central to democracy;
Giving tax breaks to college students increases college enrollment, which means a more educated population which is good for the nation;
Granting patents encourages inventions because without patents it would be impossible to make money off new inventions, so the act of the government granting special legal status encourages invention;
Giving tax breaks and marriage licenses to married couples encourages (certainly not guarantees) stable parenting and procreation which is in turn good for the nation;
This doesn't mean that the government has magically created stable families, a transparent democracy, honest congressmen, a highly educated population, and everyone is a hardworking inventor. But those things are encouraged by the act of the government granting or denying special legal benefits and protections that the population at large doesn't get. It's a question of what the government can do to encourage institutions to further various state interests.
These things aren't rights, they are privileges that the state grants to institutions when that institution furthers the interests of the state. Me paying taxes does further the interests of the state, but that doesn't mean I should be able to take the same tax deductions on my personal car that a small business owner does, or that I should be like Senators and be exempt from all laws except treason, or that I should get the same tax benefits that a married couple does.
So, in my mind, the question is what institutions provide a clear benefit to the interests of the state by being granted special legal status. IMO, straight marriage has passed that test, but gay marriage hasn't.
So in my mind the question is: how does gay marriage as an institution further the interests of a nation (keeping in mind it should provide a benefit that doesn't exist without special legal status, or will be amplified by special legal status)?