Sam2 -“Unless I am grossly misrecalling this, he doesn’t require a Mechitzah nor does he say that they can’t sit next to each other. (Maybe he is assuming that they already weren’t, but I don’t recall it saying that in the T’shuvah.)”
That’s exactly my point. He is assuming it, because if he held sitting next to or very near each other wasn’t Taaruvous -he’d say so!
In the olden times, well not that long ago, in the Frum world, not the MO world, Yeshiva dinners/ Chasunahs were almost all mixed. But people sat next to their wives, family, etc. Noone sat next to or very near, strange women/girls.
If you sit next to a strange girl/woman during a Torah Shiur, this is a classic case of “Mitzva Haboh B’aveira”!