They were fooling around in bed since they decided to spend the night together to help each other with their school project. But there was no project made, instead, clothes were taken off and rounds in bed, and other parts of his small apartment happened and he wasn't complaining.
The way Susan looked at him, he could say that she was feeling the same. He had ordered Chinese takeout for their dinner and right after, they went back to bed to fool around some more. They were going to be busy next week, thus, they were taking the opportunity to spend time together while they could. It's difficult to find the time when they both can spend the night together.