SJS, perhaps you overlooked this part:
“There is also a severe prohibition from the Torah in close friendships between a young man and woman.”
No posek disputes Rav Moshe on this. (The only question is whether it is m’doraysa like Rav Moshe paskens based on Rambam, or if it is like Ramban that rather than m’doraysa it is “only” a severe rabbinic prohibition, which even according to this shitta is still punished with rabbinic flogging.)