Like my divorce lawyer told me when I was 13 years old, don't believe anything anyone tells you until they put it in writing. I didn't believe him, so he drew up a contract and signed it, stating that a person hereby cannot take anything spoken to be truth unless the speaker has transcribed those words and signed the document in the presence of a notary public. Then he bought me ice cream.
The bottom line is that you can't trust anyone. Especially not your roommate. You two may have come to an agreement about sexile, but how do you know he'll honor it when you stumble in with a hottie at 2 am while he's 6 hours into a 48 hour Dragonball Z marathon (your roommate is a huge dork in this scenario, btw). You can't know, and that's why you need a contract.
The brand new Collegehumor Store's official Sexile (Poster) Contract is just like any other contract, but way bigger. That means it's even more legally binding.