Missouri Marriage Laws: What the State Constitution says…
1. It is the public policy of this state to recognize marriage only between a man and a woman.
2. Any purported marriage not between a man and a woman is invalid.
3. No recorder shall issue a marriage license, except to a man and a woman.
4. A marriage between persons of the same sex will not be recognized for any purpose in this state even when valid where contracted.
Missouri Statute 451.022
Source: Missouri State Constitution