Not quite. What made it scandalous was that Luther not only approved of the bigamy but asked the duke to keep it quiet because he knew it would be bad if the public found out that he had approved it.
Even Preserved Smith, on page Roman numeral IX of his collection of Luther’s letters says that Luther made two great blunders and that support of bigamy was one of them. On page 91 of the same book, Preserved Smith mentions that Luther believed a woman married to an impotent man could have an adulterous affair with another man – Smith uses the euphemistic phrase “cohabit with another.”
In the same book, Smith publishes Luther’s letter:
TO PHILIP, LANDGRAVE OF HESSE
(EISENACH,) July 24, 1540. Grace and peace in Christ. Serene, highborn Prince, gracious Lord! I have received your Grace's letter, which seems to me to have been written in a rather angry mood, although I am not aware that I have deserved your Grace's ire. For it seems to me that your Grace thinks we act in this matter to please ourselves and not, as is really the case, to serve your Grace and prevent future trouble for you. Wherefore I give your Grace to understand my real reason for advising and warning against the publication of this confessional counsel. Let your Grace not doubt that if all the devils wanted to publish this counsel, I could, by God's grace, give them such an answer that they would not get any satisfaction out of me by doing so.
For in case you publish it, I have this advantage over your Grace and all devils, too, that you must bear me witness, first, that it wag a secret confessional counsel, and second, that I have always truly begged that it be not published, and thirdly, that it will never be published by me. As long as I have these three advantages I defy the devil himself to move my pen. By God's grace I know well how to distinguish between things that should be allowed to consciences privately by way of dispensation and those which should be publicly preached. I would be sorry to see your Grace get into a war of words over this matter, for you have enough else to do. . . .
If your Grace should publish this marriage, you could not get the world to recognize its legality if a hundred Luthers and Melanchthons defended it. ...
And as to what you say about not wishing your second wife to pass for a whore, I do not see why your Grace should mind that, for she has had to pass for one hitherto, at least before the world, though we three persons and God know that she is a wedded concubine. . . .
I write these things to your Grace to show you that it is not for my own sake that I wish this matter concealed ; for if it came to a war of pens, I well know how to draw myself out of it and leave your Grace sticking in it; which, however, I would not do if I could avoid it. Nor do I think to abandon your Grace during the present crisis as long as my life lasts. . . .
Your Grace should think what an offence it would be were it published, and . - also whether you could answer for it to the Emperor, for the Bible says : " All men are liars," and, " Put not your trust in princes." . . .
Wherefore I advise you to give an ambiguous answer by which yon could remain. I commend you to God and assure you that I advise you to do exactly what I should advise my own soul.
Your Grace's obedient, DR. MARTIN LUTHKR.
I’m not certain we really want to get into the lives of medieval religious figures.
“Pope Alexander VI[1] (1 January 1431 – 18 August 1503), born Roderic Llançol, later Roderic de Borja i Borja (Italian: Rodrigo Borgia) was Pope from 1492 to 1503. He is the most controversial of the secular popes of the Renaissance, and his surname (Italianized as Borgia) became a byword for the debased standards of the papacy of that era....Of Alexander’s many mistresses the one for whom his passion lasted longest was a certain Vannozza (Giovanna) dei Cattani, born in 1442, and wife of three successive husbands. The connection began in 1470, and she bore him four children whom he openly acknowledged as his own: Giovanni, afterwards duke of Gandia (born 1474), Cesare (born 1476), Lucrezia (born 1480), and Goffredo or Giuffre (born 1481 or 1482). His other children – Girolamo, Isabella and Pier Luigi – were of uncertain parentage. Before his elevation to the papacy Cardinal Borgia’s passion for Vannozza somewhat diminished, and she subsequently led a very retired life. Her place in his affections was filled by the beautiful Giulia Farnese (Giulia Bella), wife of an Orsini, but his love for his children by Vannozza remained as strong as ever and proved, indeed, the determining factor of his whole career. He lavished vast sums on them and loaded them with every honour. The atmosphere of Alexander’s household is typified by the fact that his daughter Lucrezia lived with his mistress Giulia, who bore him a daughter, Laura, in 1492.
http://en.wikipedia.org/wiki/Pope_Alexander_VI#Mistresses_and_family