I allow my roommate to have sex with me when he sleepwalks.
That sounds horrible. Who is he? I want to know which one!
I'm not telling you his name because I don't want him to know I told you about this.
This whole situation seems weird. Are you sure he's even sleepwalking, and that you consent to this? If he's not conscious while having sex with you, it's rape. It's wrong to have sex with someone without their consent.
People can choose to sleep with me for fun, and if I'm getting something out of it, then I don't understand why this would be a problem.
You definitely have all the rights to sleep with anyone in the world but it has to be consensual.  You're not supposed to force yourself on people for sex. It's just wrong.