This is a pretty extreme position to take, because those marriages were entered into when it was perfectly legal to do so because a federal judge had struck down Amendment 3 which banned same-sex marriage in Utah.
The attorney for the lead plaintiffs in the lawsuit, Peggy Tomsic, has released a statement in response:
“This unprecedented and disappointing action harms not only my clients, but hundreds of other same-sex couples who also were legally married, and whose families have been needlessly destabilized and stripped of basic legal protection. By taking this unwarranted action, the State of Utah has discounted the lives of thousands of Utah citizens who live, work, and raise their families in Utah and pay Utah and federal taxes like all other Utah citizens. Regardless of how the State believes the Tenth Circuit will ultimately rule, these couples are legally married, and the State should treat them accordingly.”There's no question that this will result in many more lawsuits, not less, about marriage equality in Utah.
Hat/tip to Joe.My.God