Some people aren't that... well I don't want to call people stupid, but then I think of my boyfriend's roommate who wouldn't even NOTICE any type of warning (I'm saying hypothetically...no sex yet) and walk right in. I would just say if your roommates are a little on the... dull side, that perhaps you should find a friend who will lend their room to you or grab a cheapo hotel room.
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Don't be afraid to stand up for what is right!