No. You need to be legally married to file a joint return unless you are in a state that recognizes common law marriage.
STATES THAT RECOGNIZE COMMON LAW MARRIAGE
CO, DC, CO,IA,KS,MT,OK.RI,SC.TX, UT
According to IRS regulations a Registered domestic partners may not file a federal return using a married filing separately or jointly filing status. Registered domestic partners are not married under state law. Therefore, these taxpayers are not married for federal tax purposes.
Here is a link to an IRS article on frequently asked questions for registered domestic partners.
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