This Christian value of monogamous marriage was widely accepted in Europe's Christendom. Now, that many of them not believe in Christ anylonger, there's no reason not to have more than one spouse if you want. Especially in the face of multiculturalism ideology. Afterall, Islam let a man to have up to four (concurrent) wives.
I would say poligamy was practiced in the OT. It was never "accepted" by God. He never said to take more than one wife. And it should also be noted that those who were in poligamous marriages in the O.T. had all sorts of problems because of it. There isn't one account of a happy family in the poligamy of the O.T.