A decision by the Treasury Department on Thursday to treat all same-sex spouses as married under federal law has implications in all states—even those in states where gay marriage is not legal.

The Treasury Department is implementing the Supreme Court's June decision to overturn the part of the federal Defense of Marriage Act that disallowed homosexual spouses from filing their tax returns jointly.

The 130,000 or so gay married couples in the U.S. will need to use married filing jointly or married filing separately tax returns for the 2013 tax year, and have the option to amend the past three years of returns as well. Whereas before, gay spouses in states that recognized their marriages could file jointly in their states but had to file individually for federal taxes, now the opposite situation exists. In states that don't recognize marriages, couples will have to file state taxes individually and federal taxes jointly.