Remember, the decision to halt the marriages for the time being has little or nothing to do with the specific merits of the case. The case itself is not decided. When it is, it might very well still bring about wholesale overturning of state constitutional passages banning gay marriage.
If the decision eventually rules in Utah's favor, however, it would create a really bizarre scenario for the gay couples getting married there. Would their marriages suddenly be nullified? I can see the argument for holding off major changes to contracts like marriage and benefits until a clear ruling can be made.