Both polygamy and sodomy are mortal sins. Yuck!
Wrong for a Catholic, yes.
But sodomy is a perversion of nature, and one of the sins that cry out to heaven for vengeance. Polygamy is not.
In fact, many of the Fathers of the Old Covenant had multiple wives, and this was sanctioned by God. So it's FAR FROM against nature for a man to support and be married to many wives. Think about it: many children result from such a union.
One woman with several husbands -- THAT would be backwards, both because women shouldn't be "mistress" of a harem of men, and because it would result in a shrinking of the population.
Jesus Christ raised the bar on marriage (and many other things!) and that is what we must live by now. But let's not be inaccurate or exaggerate in our assessment of things.
The Mormons try to be Old-Testament, but they are very messed up in many ways. But family-values wise, at least they aren't "twisted". They're just in error.