The IRS considers all same-sex couples who are legally married to be effectively married for all tax reasons, regardless of where you live in the country. Effective in 2013 all same-sex couples who are married are subject to the same tax implication as all other married couples. The IRS will consider a same-sex marriage if it was performed in the United State Puerto Rico, territories and possessions of the U.S. or any foreign county where same-sex marriage is legal. If same-sex marriage is no legal in the state which you reside, if you marriage was authorized by the government of where it took place you are legally married according to IRS standards. If you fall under such items as registered domestic partnerships, civil unions or similar relationships that fall under state laws you are not considered married and are not subject to federal tax rules.
Rules for Married Taxpayers
Regardless of sex if you are legally married you are required to file a federal tax return as either Married Filing Jointly or Married Filing Separately. Though if you and your spouse were living separately for the last six months of the tax year, and you or your spouse had possession of a dependent child or relative you may qualify for filing as Head of Household.
If you are in a same-sex marriage and previous had to file using the status of Single you now have the ability to amend prior returns prior to September 16th, 2013 to one of the status of Married. Amended returns will be subject to a statute of limitations.