The larger issue here, which most people are completely missing, is that the Utah case will likely climb its way up to the Supreme Court no matter the outcome in the 10th Circuit, and the Supreme Court's stay is a clue into how the highest court may rule on the broader question: Is it constitutional for states to ban gay marriage?
The Department of Justice had a historic opportunity to say that since the people of these states had spoken, as long as what happens in Colorado stays in Colorado, they wouldn't make a federal case out of it. Instead, they issued a mealy-mouth statement about their ability to come in like a ton of bricks if their priorities aren't met.
DOMA clearly violates equal protection, but the Supreme Court could repeal it on more technical grounds, ruling either that House Republicans lack standing to defend it, or that it is an attack on states' rights. But either of these less desirable outcomes would still be a victory for marriage equality.