There is a difference with mormonISM. If you do not follow all the rules, become temple worthy, you are assigned to a lower level of their heaven. Romney is the highest of the high, temple worthy, in order to be there he must follow the lds line.
"Now if any of you will deny the plurality of wives, and continue to do so, I promise that you will be damned;
and I will go still further and say, take this revelation, or any other revelation that the Lord has given,
and deny it in your feelings, and I promise that you will be damned.
Brigham Young - JoD 3:266 (July 14, 1855)
Oh oh....
Romney only has ONE wife; eh?