I've seen it done before but having done it without your knowledge is kinda crappy. Before going down the path of voting authority, you might have been down the right path in starting with a conversation. The fact that you're interest is significant and they're taking actions without your knowledge is beyond concerning and should start with rewriting the act of incorporation to restrict them from taking these decisions without your signature. If there's any sort of working relationship left, they might agree if it's just a difference of opinion, and if not I'd explore selling your stake in the game. You can play all the games you want with legalities, but if their intent is to finance other interests backing it against your investments, these really aren't people you should be taking on risks with.
The main question is, did they use the corporation as proof of their own financial state and act as guarantors as individuals, or did they use the corporation as leverage for the funding? If it's the second, then you already have legal recourse as you stated having a 50% interest in the company and if the loans were for other businesses, even a board doesn't have the right to choose to have the company back debt not used for funding the company itself. This would need to go to a majority vote to either enable the board to make these types of decisions, or vote on the one off decision in itself.
If it's just a matter of the risk assessment, then again you do have a legal leg to stand on. Not for the pain in the ass that it will be, but for any costs associated. Again, a board does not have the right to force the company to incur costs related to personal endeavors outside of a special vote on the topic.
Just my 2 cents, again, best of luck.