You install the DebugEnabler package in your Mods folder in the usual way (make sure you've got the right version for your game): You can trick the game into letting your Sim be married to more than one spouse at the same time. Make sure to back up your game before trying any of the following suggestions.
The end effect is about the same, there's just no place to reliably store any other partners the Sim may have, so consistency is an issue and there is opportunity for corruption. Polygamy is not so much 'prevented through hard-coding' but there is only ONE data field that points to the Sim's 'partner'.
