I believe we have an answer to the question posed in the title to this blog post.
Scenario: A couple is in a same-sex marriage. One spouse properly filed their 2012 tax return as single earlier this year. The other spouse is on extension and hasn’t filed yet. In light of the DOMA ruling and the IRS’s just-released guidance, how should the other spouse file their 2012 return? Can they file as single, or will they have to file an amended return as married with their spouse who’s already filed?
Analysis: Revenue Ruling 2013-17 states that its provisions are effective prospectively and take affect on September 16, 2013. Any return filed by a same-sex couple after that date, whether an original return or an amended return, must use a filing status of married.*
So in my opinion, the spouse who hasn’t filed yet in this scenario could still file their 2012 taxes as single until September 16th. After the 16th, they’d have to file as married.
Read Revenue Ruling 2013-17 here.
*-Couples who would have benefited by filing as married in prior years can file amended returns as a married couple prior to September 16th if they so choose. The September 16th date refers to when couples will be REQUIRED to call themselves married on any return filed with the IRS.