An appealing "roommate wanted" post that's been up and down on Craiglist—$1200 a month, large room with terrace, near the Grand St. L train—this last week has an unusual "catch." Yeah, just a sex party once or twice a week. Don't be so uptight! Fortunately, your future roommate is both a registered E.M.T. and has a great bio online! It's endlessly fascinating.