I already made a topic about this btw
But anyways, while i don't think there is anything wrong with polygamy morally as long as it is between consenting adults, i think it would make legal marriage too susceptible to abuse of the system.
ALso, i think most practicing polygamists just want to try to have as many women as possible(This is mainly because i have never heard of a woman having multiple husbands) unlike with gay marriage where people are fighting for equal rights for gays and lesbians.