Mayhaps something like this:
Marriage shall be defined as the union of two adult human beings recognized by the United States as having the intention to remain together until death parts them. The union will only be recognized if the individuals are both in agreement that their own happiness is dependent upon the other's happiness. The individuals as a pair will receive the benefits customarily given to those in all prior uses of the terms "Spouse" and "Married"
The clergy which might officiate the ceremonial portion of such a union will retain the right to refuse to officiate, and will not be liable to any suit for refusing. The clergy may provide any reason at all for refusing to officiate without repercussion.
In the event of such a refusal, the couple may seek help in finding someone to officiate from a database maintained and operated by private citizens of sympathetic clergy.
Marriage shall never be defined as marriage between anyone not adult, anyone not human, and never more than two individuals.