Joseph, in full honesty, I was born long after Rav Moshe’s children were married, so I do not claim to have been there.
I was told by someone who said he was at both Rav Dovid and Rav Reuven’s weddings that the seating was mixed.
In any case, I refuse to believe that Rav Moshe would allow mixed seating if he felt it was not appropriate, regardless of who “the other side” was.