Why do people twist religion around to fit their culture?

It makes it appear that it is god's will. Polygamy is man's will not god's.

I like Vincent K's response.

Marriage is largely social and religions tend to be socially driven as an answer to a need for order, ethics, and morals. Unfortunately, man is far from perfect and so, gets the messages mixed up, even to the point of ignoring doctrinal points.

Using your example, if God didn't want polygamy, then why was the House of Israel born from four women (two wives and two handmaids)? The concubines of David and Solomon were condemned for being polygamous, but because David murdered to marry Bathsheba and Solomon married pagans, who eventually drew him and Israel into paganistic practices.