What happens if my fiance hasn't filed taxes in years and we get married?

You will need to talk to an attorney who is licensed n Texas.

First of all, the statute of limitations on assessing income tax does not begin to "run" until a return is filed. Then the statute of limitations begins to run. As long as you didn't sign a return you should not be personally liable for his unpaid taxes and penalties. Don't agree to filing joint returns for those years that he did not file in. You may end up having unpaid taxes taken out of any assets which he leaves to you by will or trust (if he dies). This is referred to as "transference liability." Those taxes though would simply have to be paid out of his estate prior to assets being distributed to estate or trust beneficiaries. You may want to seek advice from a Tax Attorney.

Only responsible as husband and wife going forward

You personally will not be responsible for the taxes he owed prior to your marriage. However, in a community property state, the community could be liable, which means that half of the community's income and assets would be subject to being attached by the IRS. In other words, if we assume for a moment that he is unemployed, you two are married, and you earn all the income, that income belongs to the marital community, and half of it could be attributed to your husband's old tax debt that you had nothing to do with. If he continues to not file or pay taxes on his income after you are married, you could be held liable for half of his taxes. I have had this happen just recently to a client of mine here in WA (another community property state) where she filed separately for years based on her own W-2, and her husband had a business but chose not to file. The IRS audited him, split his income in half, and assessed half of the resulting tax to her. I hope this example helps you in understanding how his unreported income can affect you after you two are married and live in a community property state.