Even though an increasing number of states are legalizing gay marriage, same-sex married couples still won’t be able to file joint federal tax returns this year. The federal Defense of Marriage Act defines marriage as “a legal union between one man and one woman,” and that’s what the Internal Revenue Service follows when it comes to federal income taxes. “Same sex couples are not recognized for federal tax purposes,” said Mark Luscombe, the principal federal tax analyst for CCH, a consulting firm based in Riverwoods, Ill. And that can make tax season a challenge for same-sex couples. “We’re absolutely hearing from folks. It’s a major concern because of the range of tax issues,” said Brian Moulton, legal director at the Human Rights Campaign, the nation’s largest advocacy group for the lesbian, gay, bisexual and transgender community.