There's much that's wrong with our income tax treatment of families. But when marriage equality advocates argue that same-sex couples face tax disadvantages because they can't marry, they don't tell the whole story. Marriage reduces taxes for couples that have one wage earner, or two wage earners who earn vastly different incomes. In other words, the "traditional" husband-as-wage-earner, wife-as-homemaker family is greatly advantaged by our tax rules. Their "bonus" for being married costs the government $30 billion. When two equal wage earners marry, they pay more taxes. So same-sex couples who marry would simply become a part of this unfair system; those with one primary wage-earner would benefit and those with two more equal wage-earners would lose out. Also, for low income parents, marriage can deprive them of the Earned Income Tax Credit (EITC); low income gay and lesbian parents who marry in Massachusestts should be grateful that the federal government doesn't consider them married!
Of course these are not arguments AGAINST allowing same-sex couples to marry. It's just that our inequitable income tax system is not a reason to support same-sex marriage either. We need to reduce the significance of marriage in the income tax system. The Alternatives to Marriage Project has the best proposals for this.