“Is that actually true? My alma mater (state flagship) has two women only dorms, and one men only dorm. There are multiple private single sex dorms as well. There are also still single sex dorms at DH’s alma mater as well.”
NU definitely has a female-only residential college. It’s very small, though, and not terribly popular. Of course there is also single sex housing on campus in the form of the Greek houses (which are on campus and owned by the university but leased by the various orgs).