text: The LEGAL reason is because it is against the law.   The question of why it is against the law cannot be answered if you do not let the answerer cite moral or religious reasons -- most criminal statutes and many civil statutes are ultikately based or moral reasons, and morality for many has ties to religion.\n\nThere are policy reasons which go beyond simple moral reasons -- the laws of our society (laws of property ownership, inheritance, parental rights, marital property, etc.) are structured on the assumption that only two people are involved in a marriage.  Allowing plural marriage creates situations which the law cannot handle.  \n\nThere are also more sociological reasons -- in practice, much plural marriage occurs when the women are very young, and it might reasonably be determined that exploitation and coercion are involved more often than free choice.  \n\nBut morality is behind all questions of "Should" or "Should Not."