In order to qualify for a subsidy, you do have to file a joint tax return if you are legally married with certain exceptions. The Health Insurance Marketplace states:
If you’re married and will file a joint federal tax return for the year you want coverage: You’re eligible for a premium tax credit and other savings if you qualify based on your income and other factors.
If you’re married and will file separately for the year you want coverage: You can enroll in a Marketplace plan together but you’re not eligible for a premium tax credit or other savings, and you may have to complete a separate application.
If you’re married and plan to file as head of household for the year you want coverage: You can say you’re “unmarried” on your Marketplace application. You’re then eligible for a premium tax credit and other savings if you qualify based on your income and other factors. See IRS rules for filing as head of household.
If you’re living apart from your spouse and are a victim of domestic abuse, domestic violence, or spousal abandonment and want to enroll in your own health plan separate from your abuser or abandoner, you can say you’re “unmarried” on your Marketplace application without fear of penalty for misstating your marital status.
*It is important to note that your insurance agent or broker cannot offer legal or tax advice and is not responsible for any tax filing liability.