Seeing as though the old Gateshead Rov Ztvk”l saw the necessity to institute separate times for boys and girls in the public Jewish shops there (see thread: ”Separate times for bochurim and sem girls in Gateshead”), at least they shouldn’t be sitting together.
They certainly shouldn’t be seated in such a way as to have unrelated members of different genders sitting next to each other.
I have heard too, although I am not sure what the source is, that if there is no separate seating at a chassuna, then according to halacha one is not allowed to say “shehasimcha bim’ono” in the zimun because there is not a proper hashra’as hashechinah such gatherings. 147, maybe this is the answer to your question – you don’t need such a hashra’as hashechinah in an aeroplane!