Generally, married couples get to file their federal income taxes jointly. Although being married can create a marriage penalty (additional tax liability) in cases of higher wage earners, it can also decrease the tax burden for others (most typically, in situations with one high wage earner and another who earns little or no income).
This tax advantage isn’t available to same-sex couples, though. Why? As you might have guessed by now, it’s because of DOMA. For federal purposes, even legally married same-sex couples aren’t . . . legally married.