I can relate to the OP's points. When a top jerks me off while he's fucking and I cum I'm also no longer horny. But as a good bottom I don't show that and let him finish anyway he wants. But when a top makes me cum handsfree by hitting my prostate when he's fucking me I found I'm still horny and can continue like nothing's happened. Even a round two (or three).
I could never figure out why that is. But because of this I never touch my cock while being fucked. I also ask the top not to touch it. Sure, some tops still do because they want to see me cum. Can't be helped. But I prefer if he doesn't and just fucks the cum out of me.
And yes, in my opinion (as BootmanLA says) sex should be fun for both, top AND bottom.