Per IRS: Married same sex couples are married but unwed same sex couples are not
In June, the United States Supreme Court in U.S. v. Windsor, 111 AFTR 2d 2013-2385, struck down section 3 of the Defense of Marriage Act (DOMA). As such, the Internal Revenue Service determined that married same-sex couples are married for federal tax purposes. However, the IRS's website continues unequivocally that same sex (and opposite sex) individuals who are in registered domestic partnerships, civil unions, or other similar relationships that are not considered marriages under State law are not considered as married for federal tax purposes. Thus, those couples are not permitted to file federal tax returns using a married filing jointly or married filing separately status.
All other Code provisions that only apply to married taxpayers similarly do not apply to registered domestic partners. They are simply not married for federal tax purposes.
Also, if the partner is dependent, he cannot be claimed as a dependent because he is not one of the specified related individuals in Code Sec. 152(c) or Code Sec. 152(d) that qualifies the taxpayer to file as head of household.
Domestic partners who reside in community property states and who are subject to their State's community property laws are also addressed by the Internal Revenue Service website. Registered domestic partners must each report their own separate income plus half the combined community income earned by the partners.