No. A marriage is a legal contract. Many businesses are owned by several different partners in a legal contract. I don't find that to be funny either..
Actually, polygamy (but not a homosexual relationship) is within the definition of the word, just not in our culture. If I remember my US history (and my school days are ancient history!) that was one of the sticking points that delayed Utah's statehood, until the LDS renounced polygamy ca. 1890.