Remember, your 2021 tax return relates ONLY to events that happened in 2021. You were not married in 2021 so you can't file married for 2021.
To answer your question, you have to have been married for at least one second in 2021 to be able file a married tax return for 2021. So, if someone were married on 31 Dec at 11:59:59 pm they are married for the entire year of 2021. If someone were married on 1 Jan 2022 00:00:01 then they are married only in 2022.
