At my (secular) university, we did have a few single-sex dorms (optional for people who were uncomfortable with a mixed-sex dorm), but all others were co-ed, though some were separated into all-male and all-female hallways where they'd share a single-sex bathroom.
IIRC even the female-only dorms had no rules about overnight stays (though males had to be escorted around the building by their female host). A university not allowing people to stay overnight reeks of puritanical values.
Essentially it's one night a week. So, if both students, effectively two nights a week.
I don't disagree. I think it would be disrespectful to your dorm mate if your partner was just living in that space (which is already small for two, let alone three) most of the time. And you have to imagine that's at least part of the reason why such things are rules now, not suggestions.