I have no idea why people think that marriage automatically creates liability for the other spouse's debts. I really like the article Is a Prenup Right for Me? until I reached this:
Third, if your fiance has a large amount of debt, you would not want to become legally liable for it. While this may sound cruel, we feel that we all need to accept responsibility for our choices in life.I cannot imagine a premarital debt that creates liability post-marriage without a voluntary act by the other party. But people still think the act of marriage by itself creates liability for debt.
There are plenty of reasons for the a prenuptial agreement (and the article has them) but this is not one of them.