Per the DOL press release:
You can find a more-detailed explanation in the DOL's Technical Release No. 2013-04, including the clarification that:
The terms "spouse" and "marriage," however, do not include individuals in a formal relationship recognized by a state that is not denominated a marriage under state law, such as a domestic partnership or a civil union, regardless of whether the individuals who are in these relationships have the same rights and responsibilities as those individuals who are married under state law. The foregoing sentence applies to individuals who are in these relationships with an individual of the opposite sex or same sex.This guidance is very similar to the IRS rule addressing federal taxes post-Windsor. This guidance is different, however, from the DOL's guidance on the Family & Medical Leave Act (FMLA), which defines spouse based on where the employee resides.
I'm coming to grips with the notion that a post-Windsor world requires lawyers to track a patchwork of federal rules, regulations, and guidance that may differ from statute to statute. I'm doing my best to keep up, but it should keep us employment lawyers busy for awhile!
Image: DOL Seal - Not official use.