Perhaps if we analyse WHY governments support the union between two people, we can figure out whether that reason requires these two people to be of a specific type, specific different types, specific colour, specific different colours, or to have any other specific attributes in common or not in common.
Are there any arguments for mixed marriages (i.e. different sexes) that do not apply to same-sex marriages? Do any of these arguments apply to all mixed marriages but no same-sex marriages?
Are there any arguments for any government support for any kind of marriage at all? Why should government even be involved?
What does or should "marriage" mean, legally, anyway? Are these people responsible for each other before the law? If so, do their individual attributes make a difference or not? Are these people still individuals? If so, why recognise any marriage by law?
Why do married couples have tax advantages? What purpose does it serve? Would it serve the same purpose if the same law applied to same-sex marriages? Does it serve the same purpose for all cases of mixed marriages? Should it apply only to those mixed marriages where it serves that purpose? Should it apply to all mixed marriages, even when it doesn't serve the purpose? If so, why should it not apply to same-sex marriages as well?