In a letter to The Jewish Press, R. Levi (Lawrence) Reisman offers the following perspective on R. Moshe Feinstein's view of mixed seating at weddings:
From what I was told, 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. Reb Reuven has acknowledged making the same accommodation for mechutonim at several of his children's weddings. At Rav Tendler's wedding, I was told, the Feinstein side sat separately while the Tendler side sat mixed.See also this post about R. Yehudah Henkin's position on the subject.
It would appear that Reb Moshe preferred separate seating, but was ready to accommodate those who felt otherwise. I know of one leader from the chassidic tradition who is sympathetic to this position. Although he will not allow mixed seating at his own simchas, he will (unlike the Satmar Rebbe, zt"l, and others) attend a wedding meal with mixed seating, as long as where he sits is not mixed.
Separate seating at weddings has a long history, supported by many sources in halacha and hashkafa. The Sefer Chassidim states that one cannot recite "shehasimcha bemeono" in Birchat Hamazon where men and women sit together because there is no real rejoicing. The Maharshal strongly endorses this position, as does the Kitzur Shulchan Aruch and the Chofetz Chaim in Geder Olam.
I know that the Levush writes that it is not necessary to be so careful with this anymore. However, the Levush can be read as stating this is the case where there is no mechitza between men and women. The original Sefer Chassidim writes of where men sit among the women; the Levush writes of where men can see the women.
I am aware that any number of rabbanim, especially in Germany but also in Lithuania, made weddings with mixed seating. I can and will admit that "mixed seating can be Orthodox"... However, separate seating has far more halachic support, and the advocates of mixed seating should keep that in mind.