From what I learned in the united States public school system in 1993, A person can only be married to one individual at a time lawfully. This may have changed more recently with muslim interests rising in the U.S. along with other group interest. The foundations of this country I believe was built mostly by christians trying to elude persicution by their own countries, therefore having the right to freely worship by their own standards and belief system. That is covered under the Bill Of Rights (freedom of religion). However, the people that write these laws had the morality of christians and popular christian belief is that an individual should be married to only one individual.