For Federal Tax Purposes, all legal marriages are now equal. Specifically, same-sex couples that are legally married in jurisdictions where their marriages are recognized will be treated as married for all federal tax purposes. This rule applies even if the couple presently resides in a jurisdiction that does not recognize same-sex marriages.
This rule comes in response to the U.S. Supreme Court decision in U.S. v. Windsor, which overruled the federal law that limited marriage to heterosexual couples. The new rule applies to all federal tax provisions where marriage may be a factor, including: filing status, personal and dependency exemptions, standard deductions, employee benefits, IRA contributions and earned income tax credit or child tax credit. This ruling does not apply to domestic partnerships, civil unions, or similar relationships recognized under state law.
As of September 16, 2013, employers must recognize same sex spouses for payroll tax purposes and implement the ruling with respect to qualified retirement plan spousal protections and benefits. The IRS indicated that further guidance would be issued on how the new rule impacts benefits and retirement plans.