No wonder he takes 2 hours to shower...
In all seriousness, the "not enjoying it" claim could be for three reasons:
1.) Let's say, hypothetically, he's actually had intercourse with a man as the bottom. He may have a had bad experience or felt ashamed from that interaction and, instead of considering it as an outlier or talking to the other person, took it as the average experience and that bottoming actually doesn't feel good.
2.) Even within the gay community, there's a lot of bottom shaming. Due to this, there is this ongoing joke that bottoms are just doing a service. Basically, bottoms are a glorified sex toy. He could have misread it as they actually take no pleasure in it at all.
3.) Again, due to shaming, it could be an unwillingness to admit it. Like, think about sodomizing throughout history. Being the bottom in a war situation or even within Greek schooling, it was the leader or the dominator topping. How, based on that history, is anyone going to take a leader that bottoms seriously? It is a sign of weakness in many cultures and people have always been more forgiving to tops because it is not too far away from what a man does with a woman. What a bottom does, though, is see as abnormal to a degree that takes away from your manhood. Even if we don't mean to, society does subconsciously show more disgust towards more flamboyant pockets of gay culture than we do more "nuclear" pockets.