So much for sola scriptura
"I confess that I cannot forbid a person to marry several wives, for it does not contradict the Scripture. If a man wishes to marry more than one wife he should be asked whether he is satisfied in his conscience that he may do so in accordance with the word of God. In such a case the civil authority has nothing to do in such a matter."
Martin Luther Luther's Letters, De Wette -- Seidemann, Berlin, 1828, vol. 2, p. 459.
So much for sola scriptura
My friend Leo Miller wrote a book called "John Milton Among the Polygamophiles," which touches on this issue.
After the Reformation, there was some doubt whether the Bible allows divorce or polygamy. The Catholic Church had insisted that neither was allowed, but Protestants were unsure for some time whether that was the word of God or the word of man.
Luther did tell one German prince that it was OK to divorce and remarry. I suspect that's what this letter may refer to. Calvin also once suggested it was OK.
Miller examines a number of dissertations written for Doctor of Theology degrees in the sixteenth and seventeenth centuries by Protestant candidates for the clergy. He shows that the issue was argued over for more than a century before it was settled.
Because of his five Divorce Tracts, Milton was primarily known on the continent during his lifetime as a divorcer and a bigamist. This was theory rather than practice: he had three wives, but the first two died of complications of childbirth before he remarried.