Avi K, I don’t have the volume and chapter reference, and I’m not going to go looking for it now, but this is what he explicitly says in multiple places: Any gathering that is open to the public needs a mechitza, gatherings that are private, meaning that people can attend only by invitation, do not. Thus, he says, a wedding doesn’t need one, but a shiva house does.
By the way it is not true that all his children’s weddings were mixed. He did whatever the mechutonim wanted, so two were completely separate, one was mixed, and one was mostly separate with some mixed tables.