Yes, the law is outdated and needs to be refined, but there are many reasons why it has stood solidly in modern times. 

Polygamy is not the same as polyamory. If you don't know what the difference is - google it.

Polygamy relies on obedience, modesty, and chastity codes that bar women from operating as full citizens and enjoying their civil and political rights. You can't watch one episode of Sister Wives and say "but they're happy, look at them!" They are the 0.1% of what these unions look like. 

In any other situation, these women would be tucked away in a small town homeschooling their kids. They are cut off from all mainstream media. Kids grow up knowing no other option. Boys grow up competing for wives and are kicked out of the community if they can't get a wife - because there is that much competition. 

Read "Under the Banner of Heaven" by Jon Krakauer, it's fascinating but also extremely disturbing. 

Also read:
http://www.justice.gc.ca/eng/rp-pr/other-autre/poly/chap1.html