At Brown, there's one single-sex HALLWAY. That's called "the convent", specifically for girls whose parents request that their daughters stay in single-sex living arrangements.
Other than that, it's a free-for-all. Rooms and suites have to be single-sex, but we're co-ed by hall, meaning you can live next to someone of the opposite sex. I never saw a problem with it--there weren't that many hook-ups. Lots of couples, but not so many hookups.
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