Common law spouses in the states that recognize these marriages are considered married for federal tax purposes.
Registered domestic partners are considered unmarried and must file as single if they don't qualify for head of household status.
Head of household status provides for a larger standard deduction and wider tax brackets, at least at low and moderate incomes.
Yes, you're still technically married—neither of you is free to marry anyone else—but not for tax purposes.
The key word is "legally." You and your spouse cannot simply move into separate households or reach a separation agreement between yourselves unless that separation agreement is made into a court order.
This applies to domestic partners in the states of Washington, Nevada, and California.
At the federal level, persons in valid domestic partnerships or civil unions must file their federal tax returns using either the single filing status or head of household.
The IRS states in Revenue Ruling 2013-17: "For federal tax purposes, the terms 'spouse,' 'husband and wife,' 'husband,' and 'wife' do not include individuals (whether of the opposite sex or the same sex) who have entered into a registered domestic partnership, civil union, or other similar formal relationship recognized under state law that is not denominated as a marriage under the laws of that state." Some states require that registered domestic partners and persons in civil unions must file state tax returns as if they were married.