I can concede the point: it's not fair that marriage should be defined as one man and one woman. But who said life is fair?
The reason societies recognize and reward certain relationships is to have MORE of them. It is in our best interest as a civilization to encourage males to stay with the mother of his children. It's better for children to have their mom and their dad, and it creates less of a burden on the rest of society. If males just NATURALLY chose to stay with and be faithful to the mother of their children, frankly, there would be no need to have a formal recognition of their union.
But on Earth, males have no such natural proclivity...
While some animals are wired up to mate for life (swans and French angelfish for example), humans are not. If it happens in humans, it happens BY CHOICE.
One way that society entices males to make that choice, is by rewarding marriage with tax-breaks and other benefits. Civilizations which offer an incentive to males for fidelity and legal responsibility of their offspring have fewer fatherless homes, greater productivity and less poverty.
By contrast, it's not necessary to incentivize marriage for females, because women are naturally wired up to be fiercely protective of their offspring, and extended families. They are already wired to stay.
I don't mean to imply that Dads don't love their kids. They certainly do! But sometimes love isn't enough. And sometimes Dad doesn't stick around long enough after conception to develop a love for his children.
What does this have to do with "marriage equality"? Simply this: if the reason people get married is no longer to "stabilize families for the good of us all" but "because they're in love," then it will, by default, create more fatherlessness.
After all, if ANY two people are perceived to be equally adept at raising children -- in other words a male parent brings nothing unique to child-rearing -- then the incentive for a male to choose to be faithful, as well as legally responsible for his children, disappears.
A man would be justified in believing that the mother of his children can hook up with any other partner (male or female) and produce the same results. If he believes his particular connection to his offspring isn't essential for their well-being, then why bother to make the effort to go against his nature?
"If the law redefines marriage to say that fathers are optional, then it's hard to make the argument that fathers are essential." ~ Ryan T. Anderson