Assuming all the women in the marriage are of age, and there is consent, why isn't polygamy legal? Judaism, Islam and, apparently, some sects of Christianity consider it to not be contrary to their beliefs. If two men consent to marry each other and everyone cries that they should be allowed to, then certainly two or more consenting women and one male should be allowed to aswell. It seems to me that this maybe a) a restriction on religious rights and b) the imposement of one group's religious beliefs on others.
(Please, keep recent current events out of this discussion, as they involved serious abuse of minors, hypocrocy, etc and are not relevant here.)