Amount of income tax paid tends to be less for a married couple, verses if the couple files separately. The exception being, if both individuals separately have enough income to be in the same high tax bracket.
I don't think you have to be married to jointly file taxes, though.
Source: Not an IRS accountant.
I'm pretty sure you do have to be married (or domestic partnership) to file jointly. You may be thinking of Head of Household tax rates that are better than single filers'. There's :
Single (or Married filing separately)
Married filing jointly
Head of Household
Look at the tax rates - Head of Household isn't as big of a savings. For example, the 22% bracket.
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