It's sexual submission which drives me to suck cock so I also don't need my own sexual relief to feel satisfied. It's exciting, thrilling, and erotic...
I've had them and I've had guys fuck me and miss stimulating my prostate; I have a prostate stimulator that, well, it vibrates with differing speeds...
I don't think it's unusual and I believe that it's because once you get to sucking, your focus is on what you're doing to him, which is arousing you...