Would a post-nuptual agreement be enforced as a way to protect my assets after getting married?

Did you sign a joint return? If so, no. If not, then a post nuptial contract may work. A formal legal separation may also achieve your goal. A divorce would also isolate your assets to the highest degree possible. The tax authorities have immense collection authority so doing something as tax avoidance can be illegal; tax planning is legal. Any lawyer will want to research this thoroughly before beginning. I would suggest you consult with a tax lawyer first then hire someone to do the post nuptial agreement or whatever is needed. I helped a couple in similar circumstances but it did not involve taxes.