My first orgasm was the result of anal stimulation.
I was in my early teens and messing about in the shower. Touching my asshole felt pretty good. One time I brought a smooth plastic 3-4 inch slender thing into to the shower (I don't remember what it was, a small piece to something). I crouched down and slid the piece into my ass. It felt good. Then I wiggled it around in my ass and it felt VERY good. I kept wiggling it around and it felt better and better. Then suddenly and unexpected to me this white goo shot out of my cock in spurts and I felt the greatest sensation of pleasure that I have ever felt at that time in my life wash over me.
The rest is history.

I will never forget that feeling both mental and physical.

Since then I have never cum just from prostate stimulation but I have come very close and my orgasms are always stronger and more pleasurable as a result of prostate stimulation combined with stroking my cock.