Gay couples and straight couples are inherently not equal in value to society. It is not unequal to treat things of unequal value unequally.
In order for the federal government to recognize marriage it must first define what marriage is.
But all couples are inherently not equal in value to society. If you have a stable gay couple who adopt an unwanted child and raise it to be a productive member of society, by what possible grounds can you make an argument that couple is of less worth to society than the hetero couple who raises a gaggle of criminals on welfare, or the childless hetero couple who save not a penny and retire on taxpayer dollars, or the hetero couple where he beats the crap out of her twice a month and they're in court every month? Evaluating a couple's actual worth to society would require setting values on everything we do and practically speaking would be impossible. Is a rich man whose wealth is gained entirely on day trading (one winner, one loser, no net change in wealth) worth more to society than the suicide prevention counselor who earns minimum wage but saves lives? What about the big city man who stamps out widgets all day, actually producing a net societal increase in wealth via manufacturing but never earning enough to pay in taxes his own cost in government services?
Frankly, the very idea of government setting out to determine one's legal privileges based on one's value to society seems to me far too Orwellian to be a good idea. Much better to treat everyone the same unless there is some compelling reason to do otherwise. As for gay couples, anything bad in having open homosexuality we already have; going back in time is simply not an option. The societal benefits of marriage, on the other hand, are exactly the same regardless of whether one's spouse is an innee or an outee, has matching private parts or mirrored private parts. It truly baffles me that people who honestly believe that homosexuality is a significantly negative societal force, having lost the war, are still fighting the last battle against what by any measure are positive societal benefits. For a small government fiscal conservative or libertarian, is it not good that every family can keep more of what it earns? For a social conservative or traditionalist, is it not good that everyone can choose to embrace what we advocate, marriage and family? THIS is the battle we're fighting now, not whether homosexuality is to be accepted in our society but whether or not homosexuals are to be afforded the same basic rights we enjoy and which we insist strengthen society. That we're fighting this battle at all, considering that there is no practical detriment to non-homosexuals, is just baffling. This isn't homosexual teachers or homosexual Scout Masters or homosexual priests or anything else even indirectly affecting others, it's merely about legal protections and the legal relationship between two adults and their government. There is no down side.