Marriage is actually not defined in the Texas Family Code. It certainly isn't considered a contract as a normal contract is viewed under contract law here in Texas. What is the reason for the question?
Prenupts and other documents signed by the parties might be considered contracts, and yes a marriage license is signed by the parties, but no contract in and of itself.
Disclaimer: This answer is intended for informational and educational purposes only and should not be considered legal advice nor forming the attorney client relationship.