during the 1800's particularly the industrial revolution, the average life expectancy of a male was 15 years old girls I believe was 18 or so, women also developed faster in previous times hence why Julliete is 16, and why Edgar allen poe married a 13 year old while edgars was a bit to an extreme it was much more common then. They could bare children and they got married at that time, while I find it quite disgusting now, if odds are you could die before you hit 18 wouldn't you marry young? also from a strictly survival point of view the polygamy thing was a very smart move by the LDS church to increase its numbers it also occured in the old testament, which means it also occurs in the Koran, and the Torah while I am not Mormon nor do I support Plural marriage in modern times it made some sense at the time.
not that you really care about facts or anything, which mine might be off a bit on some of the numbers. its all from the top of my head.