From the catholic website:
Philip the Magnanimous (b. 23 Nov., 1504) was married before his twentieth year to Christina, daughter of Duke George of Saxony, who was then in her eighteenth year. He had the reputation of being “the most immoral of princelings”, who ruined himself, in the language of his court theologians, by “unrestrained and promiscuous debauchery”. He himself admits that he could not remain faithful to his wife for three consecutive weeks. The malignant attack of venereal disease, which compelled a temporary cessation of his profligacy, also directed his thoughts to a more ordinate gratification of his passions. His affections were already directed to Margaret von der Saal, a seventeen-year-old lady-in-waiting, and he concluded to avail himself of Luther’s advice to enter a double marriage. Christina was “a woman of excellent qualities and noble mind, to whom, in excuse of his infidelities, he [Philip] ascribed all sorts of bodily infirmities and offensive habits” (Schmidt, “Melancthon”, 367). She had borne him seven children.
The mother of Margaret would only entertain the proposition of her daughter becoming Philip’s “second wife” on condition that she, her brother, Philip’s wife, Luther, Melancthon, and Bucer, or at least, two prominent theologians be present at the marriage. Bucer was entrusted with the mission of securing the consent of Luther, Melancthon and the Saxon princes. In this he was eminently successful. All was to be done under the veil of the profoundest secrecy. This secrecy Bucer enjoined on the landgrave again and again, even when on his journey to Wittenberg (3 Dec., 1539) that “all might redound to the glory of God” (Lenz, op. cit., I, 119). Luther’s position on the question was fully known to him. The latter’s opportunism in turn grasped the situation at a glance. It was a question of expediency and necessity more than propriety and legality. If the simultaneous polygamy were permitted, it would prove an unprecendented act in the history of Christendom; it would, moreover, affix on Philip the brand of a most heinous crime, punishable under recent legislation with death by beheading. If refused, it threatened the defection of the landgrave, and would prove a calamity beyond reckoning to the Protestant cause.
Evidently in an embarrassing quandary, Luther and Melancthon filed their joint opinion (10 Dec., 1539). After expressing gratification at the landgrave’s last recovery, “for the poor, miserable Church of Christ is small and forlorn, and stands in need of truly devout lords and rulers”, it goes on to say that a general law that a “man may have more than one wife” could not be handed down, but that a dispensation could be granted. All knowledge of the dispensation and the marriage should be buried from the public in deadly silence. “All gossip on the subject is to be ignored, as long as we are right in conscience, and this we hold is right”, for “what is permitted in the Mosaic law, is not forbidden in the Gospel” (De Wette-Seidemann, VI, 239-244; “Corp. Ref.”, III, 856-863). The nullity and impossibility of the second marriage while the legality of the first remained untouched was not mentioned or hinted at.
His wife, assured by her spiritual director “that it was not contrary to the law of God”, gave her consent, though on her deathbed she confessed to her son that her consent was feloniously wrung from her. In return Philip pledged his princely word that she would be “the first and supreme wife” and that his matrimonial obligations “would be rendered her with more devotion than before”. The children of Christina “should be considered the sole princes of Hesse” (Rommel, op. cit.).
After the arrangement had already been completed, a daughter was born to Christina, 13 Feb., 1540. The marriage took place (4 March, 1540) in the presence of Bucer, Melancthon, and the court preacher Melander who performed the ceremony. Melander was “a bluff agitator, surly, with a most unsavoury moral reputation”, one of his moral derelictions being the fact that he had three living wives, having deserted two without going through the formality of a legal separation. Philip lived with both wives, both of whom bore him children, the landgravine, two sons and a daughter, and Margaret six sons.
The marriage in spite of all precautions, injunctions, and pledges of secrecy leaked out, caused a national sensation and scandal, and set in motion an extensive correspondence between all intimately concerned, to neutralize the effect on the public mind. Melancthon “nearly died of shame, but Luther wished to brazen the matter out with a lie” (Cambridge Hist., II, 241).