No.  In the United States, various states established domestic partnerships and civil unions by law to grant quasi-marriage rights within those states to same-sex couples in response to the 1996 federal Defense of Marriage Act (“DOMA”) and state prohibitions on same-sex marriage.  The particular privileges granted to these couples depend on the law of each state granting them.

In 2004, Massachusetts became the initial state to legalize same-sex marriage, with sixteen states (and eight Native American tribal jurisdictions) following.  Later, in 2013, the United States Supreme Court struck down DOMA’s Section 3, which mandated federal non-recognition of same sex marriages, in United States v. Windsor.  After the Windsor decision, the IRS issued Revenue Ruling 2013-17, to communicate its recognition of the legitimacy of same-sex marriages entered into in a state whose laws authorize the marriage for federal tax purposes.

However, the IRS determined that individuals who have entered into a “registered domestic partnership, civil union or other similar formal relationship under state law that is not denominated as a marriage under the laws of that state” will still not be treated as married for federal tax purposes.  Thus, in order to be treated as married by the IRS, a couple must truly be married.