I am sure that this has been mentioned already in this thread: The way R’Moshe learns the sugya of mechitza, one is required for a wedding, even for seating, just like one was needed by Simchas Beis Hashoeva and just like one will be needed by levaya of Mashiach ben Yosef.
Others learn the sugya of mechitza as applying only to shuls and therefore opening up the possibility of allowing mixed seating at weddings.