It depends. Texas is one of a handful of states that still recognizes common law marriages. Therefore, if you meet the statute of a common law marriage, then, yes, you may file a return as Married Filing Joint.
Remember, though, that this does make you married in Texas, which also puts you under the Community Property Laws.
Here is a useful link for additional information. Please review each website listed:
Texas Common Law Marriages
@dougwallace1964
**Say "Thanks" by clicking the thumb icon in a post
**Mark the post that answers your question by clicking on "Mark as Best Answer"