Inasmuch as laws have been enacted by Congress forbidding “future” plural marriage...
The Congress has no power to make law retroactive to a time before the law was enacted.
So, those marriages sealed in the Temples for time and eternity prior to the 1883 Edmunds Act and the 1887 Edmunds Tucker Act would remain legal and lawful.
So much for the secular explanation - now, how about the LDS one?