Why can't anyone just admit that this dude is just a pregnancy obsessed breeder? He literally bottoms out one of his wives wombs until they are loose, fat, and ravaged by years of back-to-back birthing then goes out and finds a younger woman to marry.
I mean reading the comments everyone tends to parlay it up to religion/culture/choice but reading Kody's comments about "Meri's fertility" and "when can we try for a baby?" to his new wife, it's obvious that this dude just has a serious case of:
http://en.wikipedia.org/wiki/Pregnancy_fetishism