I read somewhere in the Igros Moshe that a mechitzah is required at every wedding. Does anyone know where in the Igros Moshe it says that the reason we have a mechitzah at a wedding is to prevent kalus rosh?
While there are no clear Teshuvos on the matter, R' Lawrence (Levi) Reisman wrote a letter into the Jewsh Press where he noted that:
..two of the four weddings Reb Moshe made had completely separate seating. At Reb Reuven’s wedding, there were some mixed tables to accommodate those from his wife’s side who wouldn’t sit any other way..
..It would appear that Reb Moshe preferred separate seating, but was ready to accommodate those who felt otherwise..
See here for a longer excerpt.