@SuperLutheran @shortstories I think this guy has a pretty decent article about the quote: https://beggarsallreformation.blogspot.com/2009/01/luther-i-confess-that-i-cannot-forbid.html

The quote apparently comes from a volume of Luther's letters put together by a Dr. Wilhelm Martin Leberecht de Wette.

The letter is Luther advising Chancellor Gregory Brück on how to proceed as the Chancellor was approached by a man who wished to take a second wife as his then wife was ill and unable to perform her "marital duties".

The original German of the letter found here: https://books.google.com.au/books?id=pDcRAAAAYAAJ&pg=PA459&redir_esc=y#v=onepage&q&f=false

Full context of the quote:

The husband must be sure and convinced in his own conscience by means of the Word of God that it is lawful in his case. Therefore let him seek out such men as may convince him by the Word of God, whether Carlstadt, or some other, matters not at all to the Prince. For if the fellow is not sure of his case, then the permission of the Prince will not make him so; nor is it for the Prince to decide on this point, for it is the priests business to expound the Word of God, and, as Zacharias says, from their lips the Law of the Lord must be learned. I, for my part, admit I can raise no objection if a man wishes to take several wives since Holy Scripture does not forbid this; but I should not like to see this example introduced amongst Christians. ... It does not beseem Christians to seize greedily and for their own advantage on every thing to which their freedom gives them a right. . . . No Christian surely is so God-forsaken as not to be able to practice continence when his partner, owing to the Divine dispensation, proves unfit for matrimony. Still, we may well let things take their course" [To Chancellor Bruck, "Brief wechsel," 4, p. 282: " Oportere ipsum maritum sua propria conscientia esse firmum ac cerium per verbum Dei, sibi hcec licere."] [Hartmann Grisar, Luther Vol. 5 p.72]

The way I read it, Luther is saying he cannot forbid it as he does not see Scripture forbid it and so considers it a matter of Christian freedom. However, he also says that even though a Christian may have the liberty to do this it should not become widely practiced and should only be considered in certain very rare cases.

I fall pretty clearly on the "two become one" approach to marriage and so would forbid polygamy, but given the context that Luther is in, the importance of heirs, and the power wielded by some of the men who wished to take a second wife I understand his argument even though I disagree with it.