In its recent United States v. Windsor decision, the Supreme Court decided that the federal government had to recognize same-sex marriages in states that allow them. That leaves people in the seven or so states that have civil unions but not marriage in a bit of legal limbo. Right now, it looks like it's going to be up to the executive and the various federal agencies to determine how to treat these people.
The thing about being in legal limbo is that it practically begs for a court challenge. Is separate-but-equal okay when it comes to marriage? The Supreme Court could have decided that as part of their ruling in the Prop 8 case, but they chose to instead deny standing, booting it down to the state level and sidestepping the issue entirely.
That means that separate-but-equal partnership/union laws are still ripe for legal challenges along two main avenues.
- Can the federal government refuse to recognize state-backed civil unions that grant the same rights as marriage under federal law?
- Can states create separate-but-equal institutions at all? Is that a violation of the Equal Protection clause? Does it pass whatever level of scrutiny (strict or intermediate) is appropriate when evaluating such laws?
I'm a firm believer that (2) is unconstitutional, but I'm not a Supreme Court justice. And if (2) is struck down, (1) is sort of implied. But I think that you could come up with a situation where (1) is found to be unconstitutional while (2) is not based on the duck principle: "If it looks like a duck, walks like a duck, and quacks like a duck, it's a duck."
In other words, since the federal government has no legal notion of civil unions or domestic partnerships, then couples are either married or not married under federal law. If a state allows a couple to enter into a legal relationship indistinguishable from marriage except for its name, then the federal government is obligated (under the same principle as in Windsor) to recognize it as such.