Polygamy just doesn't work in a modern society. The natural balance of men:women is too even (more men even).
It only works in primitive societies as there lots of men have a habit of dying in various work accidents or wars more than women do.
I'll agree that it could maybe be done after a big war...but how could you get a conventional war so big?
Thinking further...I suppose it could work if its very tightly controlled. Say Islamic style only harder, only in very certain situations- i.e. your brother dies then you take his wife or somesuch.
Hmm, I know this is the pre-1900 forum, but
This does strike me almost impossible without either a very early POD or a very late POD (modern secular society ).
POD 1983ish Louis Farrakahan, in a response to the Genocide of crack and prison inflected upon the Black Race by the American goverment reintroduces polygamy in his sect.
(I actually saw a article in Essence or Jet about sharing because of the low numbers of "eligable black men", caught my eye on the cover while checking out hot girl

).
Fears of being called racism prevents early crushing. Slowly sprend through black community for a couple years with certain high profile sports and music people popularizing it.
Finally high profile case, generating huge scandal. Press condemnation weakened by accusantions of racism.
Defendants win the court case (activist judge? religious freedom? comparison with other blended families?)
Ruling written in such a way to allow secular polygamy or quickly followed up by non-religious polygamist claiming unfairness.
Say around 1992?
THoughs?