And the more fun for them. I know one guy who deliberately tries to hold his cum back so that he can frustrate his regular cocksucker, who gets wilder and more desperate in his efforts to make him cumm. And when he finally cumms, the cocksucker is so happy he always tells him he's grateful to get his cum, and immediately asks when can they meet next. Sounds exciting to me. I'd love to watch them together sometime.
Make your Cocksucker struggle to get it out of you. Make him work. It makes him hotter and more determined when it seems like a cock is successfully resisting him. Bring out his best. You'll enjoy it more and he will too. Cocksuckers love a struggle and their victory is proven when they get that cock to cumm!