The best answer I can give you is "maybe."  Texas does recognize informal marriages, also known as common law marriages.  To determine if someone is common law married, the courts use a three part test:  (1) cohabitation, (2) an agreement to be married, and (3) a "holding out" of the marriage.  "Holding out" is basically where you tell other people that you are married.  So, the filing of joint taxes, by itself, does not make you married.  However, it can be evidence of a common law marriage.  If you filed taxes together as 'married', but did not intend (i.e. agree) to be married, then you are not married.  If you filed taxes, have told people that you are married, and you, in fact, did agree to be married, then you are considered married.