I walked in on my roommate stroking his cock.  Instead of looking embarrassed, like I would have, He just kept stroking, while looking me directly in the eyes.  My eyes flicked down to his hand, moving up and down on his thick meat.  He saw this and smirked.  After a few more jerks, he let go of his dick and gave me the finger.  His cock stayed hard and twitched a couple of times.  My eyes never wavered, and I may have unconsciously licked my lips.  I heard his deep voice command me to stop staring and start sucking.  Something about the forcefulness in his tone, made me obey immediately.  In five minutes I went from his friend and roommate to a hole for him to use for pleasure.

I never walked in on him jerking off again, as he never had to jerk off again.  For the rest of the semester he used my mouth or my ass to stroke his cock.  

Saved on the laundry costs as well, since he no longer needed to wipe his cum off his flat stomach and muscular chest.