The mental build-up psyches you out, often leading to a heightened state of arousal. Think about it: entering a room where a man is bent over a bed, bottle of lube to help your penis enter his ass, the sight, sounds and maybe the smell of the anal penetration, not to mention how tight his asshole might be. DEfinite recipe for an early and intense orgasm that would, for most people, produce a ton of semen. And the head of that guy's cock is enormous, must've really pressed against the bottom's prostate.
Doesn't always have to be a long fuck. He slides his erect penis into your rectum and proceeds to fuck you until you feel him cum in you as he pumps his seed deep into your willing submissive body where you both want it and now that it belongs in there.