I posted TX law in one of these thread. Texas does legally recognize common-law marriages.
Common Law Marriage or Informal Marriage:
Texas also refers to common law marriage as an informal marriage.
For a marriage to be declared an informal marriage in Texas, a couple has two options.
1. Sign a declaration of their marriage under oath. The form is available at County Clerk's office.
The Declaration and Registration of Informal Marriage asks for full names, woman's maiden surname, addresses, dates of birth, places of birth, social security numbers, and relationship information.
The Declaration states: "I solemnly swear (or affirm) that we, the undersigned, are married to each other by virtue of the following facts: On or about (Date) we agreed to be married, and after that date we lived together as husband and wife and in this state we represented to others that we were married. Since the date of marriage to the other party I have not been married to any other person. This declaration is true and the information in it which I have given is correct."
2. Live together as husband and wife in Texas
Represent to others that they are married.
Agree with one another that they are married.
Individuals under the age of 18 may not enter into an informal marriage.
http://marriage.about.com/cs/marriagelicenses/p/texas.htm