How could his butt give him such pleasure? No, it might not be right to say that it was his butt that gave him pleasure because it was actually Wu Jinhai who did such a great job on his ass that managed to make him feel like he was flying in the sky when he reached his climax.
Even his experience with Zhu Ge when that man gave him a hand job could not be compared to what he had just tasted, and he was really curious about what was inside his ass that managed to give him such pleasure.
Unfortunately, he was not a good student when he was in school, and because of that, he didn't pay much attention when his teacher was explaining biology. He foolishly thought that there might be something like a button inside his ass, and Wu Jinhai happened to touch it with his penis, and that was the source of his pleasure.