It is a little more nuanced than that. Massachusetts legalized gay marriage before the feds. LGBTQ couples in Mass. couldn't file jointly because the feds didn't recognize their marriage as legal, even though the state did. She's asking for back taxes to be paid to those specific couples that were married legally in the state of Massachusetts before it became legal nationwide.
“The federal government forced legally married same-sex couples in Massachusetts to file as individuals and pay more in taxes for almost a decade,” Warren said in a statement,
according to NBC News. “We need to call out that discrimination and to make it right — Congress should pass the Refund Equality Act immediately.”
"It wasn’t until marriage equality became law that gay & lesbian couples could jointly file tax returns—so they paid more in taxes," the Democratic presidential candidate posted to Twitter on Sunday. "Our government owes them more than $50M for the years our discriminatory tax code left them out. We must right these wrongs."