Luther didn’t attend the wedding. Philip married Margarethe von der Saale on March 4, 1540, at Rotenburg an der Fulda. The Wittenberg theologians present were Melanchthon and Martin Bucer.
His real involvement came a few months earlier, in December 1539, when he co-signed the Wittenberg Advice with Melanchthon and Bucer. They framed it as confidential pastoral counsel, a dispensation under the seal of confession. When the secret got out anyway, Luther didn’t defend polygamy on principle but suggested Philip deny it, the infamous “good strong lie” remark.
On “preferable to divorce,” that’s fair. He said something like it in The Babylonian Captivity (1520) and again around 1531 regarding Henry VIII and Catherine. But that fits the Matthew 19 point: he was weighing two things he considered falls from the creation pattern, not calling plural marriage good.
I’d call it practical rather than a Reformation-era view. Nothing like this made it into the Lutheran Confessions. Melanchthon got so sick with guilt afterward that Luther had to pray him back to health.