At one time, of course, same-sex partners were not legally defined as married couples in any state of the country. Given that, many people found questions relating to same-sex couples and linked issues surrounding federal taxation to be both incomprehensible and irrelevant.
That is certainly not true anymore, with consistently evolving social mores and legal adjustments resulting in new definitions and legal rules/standards regarding the meaning of family and related matters.
In 2013, the United States Supreme Court took a hard look at the federal Defense of Marriage Act in a case called Windsor v. U.S. and found it wanting.
That legislation sanctioned marriage only between heterosexual couples, something that court justices found indefensible under the Constitution. The act was thus invalidated.
Not all states fell in line with Windsor, with some still holding out marriage as being lawful only between a man and a woman. Such lack of unanimity rendered tax-related issues complex and inconsistent across the country.
Two years later, the court followed its Windsor ruling with one of equal import in the 2015 case of Obergefell v. Hodges. The court ruled in that seminal case that same-sex marriage is legal in every state of the country.
The IRS has followed through in Obergefell's wake with clarifying regulations and language of its own. The agency recently issued so-called "final regulations" holding that marriage is marriage for federal tax purposes, whether it is a union between opposite-sex or same-sex couples. There is no distinction, period.
New IRS rules focused on that subject matter and related considerations were published earlier this month. They amend a number of regulations, including those concerning income taxes, gift taxes, estate taxes, employment taxes and generation-skipping transfer tax regulations.
Questions can be addressed to an experienced tax attorney.