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 …”
Perhaps that refers to the chuppah itself (though in outdoor chuppahs, people all stand around together, mopre or less in my experience). Maybe if the chuppah is “tainted” by this, the zimun later on is also considered to be tainted. I have never heard of such a thing, however. Hashem created male AND female. It hardly seems likely that He would remove His Shechinah because men and women were in the same place together being mesameach with a Chosson and Kallah.