Your girlfriend's debt may mean she also has poor credit. In that case, you'll be affected if you sign up for joint accounts or if you try to make a large purchase that requires a loan. So if you want to buy a house after you get married, and you apply for a mortgage, the debt-to-income ratios and credit scores are factors they consider determining the loan amount and interest rate. If her credit is that bad, you may not want her on the loan app, but at the same time, you also may not be able to qualify for the loan with your income alone. You could always consider renting.
