Basically as soon as they were of child-bearing age, very often to considerably older men ... was considered perfectly normal and reasonable ... 'polygamy' as such had nothing to do with that phenomenon. It was simply a case of men thinking that by starting w/a young female, they would, in the end, end with up with more progeny overall. And of course by 'starting young', they were unlikely to have to care for another male's offspring from a previous marriage or at least pregnancy.

Was it 'cool'? Of course not. It was downright 'primitive' and animal-like behavior. But it was a very different time, and human's lives were much closer to those of animals. The decision seemed reasonable and normal to them. And if a girl was brought up their whole life to believe they'd be married at 12 or 13, and all the girls around them experienced the same thing ... I doubt it had quite the same negative psychological impact it would have to someone growing up in our present, western society, experiencing the same thing.

It's easy to 'project' our way of thinking onto people of the distant past, but it's probably quite often inaccurate. We don't have much clue how brutal 'life' was for people hundreds of years ago. Being a 13 y.o. girl married to a 30 y.o. man was not necessarily the hugely traumatic 'thing' we imagine. Fact is, you had a lot more serious 'stuff' to worry about back then.

Again,not saying it was 'right' (and is obviously TODAY ... wildly inappropriate and indefensible, given modern life and our level of knowledge, and mobility), but ... 200 years ago life was way different than nowadays. To US, 'older dude was a perv' ... to them ... it probably seemed a reasonable calculus. Some of them undoubtedly were 'pervs' by our 21st Century definition. Others just wanted a ton of kids.