I imagine that this is how it went ...
You and your roommate watched a porn movie. He told you that the actress had the most beautiful ass he'd ever seen. You told him that you had the "perfect" ass. He started laughing and didn't believe you. "There's no way your ass could be prettier than hers", he told you.
You said to him : "Shall we bet?" And you added : "The loser of this bet should do what ever the winner wants". He thought about it for a few seconds, before agreeing to the terms of the bet. You told him you were going to the bedroom and that he had to wait ten minutes before coming to the bedroom.
Ten minutes later he walked to the bedroom and he knocked on the door. You said that he could come in. When he walked into the room, he saw you lying on the bed. You were wearing black lingerie. You turned around and you said : "Take a good look and tell me what you think".
He said : "Wow ... you are right, your ass is perfect". You replied : "I told you so". "So you agree with me that you have now lost the bet", you added. He nodded his head and asked what you wanted him to do.
You told him to drop his pants. He obeyed and you noticed that he had a hard cock in his boxers. You got off the bed and crawled to him on your hands and knees. The closer you got to him, the bigger his cock in his boxers got. You kissed his cock before you pulled down his boxers. You started teasing him, with your tongue. Slowly you moved your tongue over the top of his hard cock.
Then you took his cock in your mouth and started sucking it. You loved every minute you had his hard cock in your mouth. And you could tell by his breathing that he also enjoyed this blowjob. You took his cock out of your mouth and saw that he was ready. You got up and walked over to the bed. You let your panties slide down and got down on your hands and knees on the edge of the bed.
Then you said to him in a soft tone : "I want you to fuck my ass". He walked over to the bed and he caressed your buttocks. Then he pressed the tip of his hard cock against your anus. "Be gentle with me", you whispered. Softly his hard cock went inside your ass and he started to thrust your ass gently.
Slowly he began to thrust harder and deeper inside you. You could now feel every inch of his cock in your ass. And moaning softly you enjoyed every thrust inside you. You could heard him moan louder and louder now and you knew he was about to cum. You told him to stop and that he should get his cock out of your ass. You turned around and again you got down on your knees on the floor.
You took his cock back in your mouth and you started sucking it. A few seconds later he blew his load into your mouth. You now opened your mouth so that he could see his cum in your mouth, before you swallowed it.
He may have lost the bet ... but he also won the grand prize. In the end you both got what you want ...