I'm no doctor but;
Most of the ejaculate ('Cum') is liquid from your Prostate gland where the sperm are mixed into the carrying fluid made in the Prostate. The 'VAS' brings the little swimmers up to the prostate just under your bladder where they mix with the fluid made in the prostate, and then when the orgasm happens you shoot it out. You can orgasm even if you don't cum for example men who have had their prostate removed altogether for prostate cancer. its a state of mind thing. The idea that swelling balls full of thick juicy cum await the porn film belting out of your cock in hot heavy jets is erm -.. bollocks... because not very much of the ejaculate or spunk originates in the balls in fact.
That's also why men who have had a vasectomy can still spunk - nothing comes from the balls at all 'cos the tube is cut or sewn up, but the cum is made in the Prostate so you still spunk.
FF
You're pretty well 100% correct. Around 10% of ejaculate is direct from the balls, the other 90% is as you correcltly say, from the prostate.
As for the state of mind thing, hmmmm. Guys who have had their prostate removed are lucky if they can still have the sensation of orgasm, it's very hard for the surgeon to remove all the prostate tissue without wrecking the nerves involved in the male orgasm.
And you are right about vasectomised guys losing only a little of their cum, as above, about 10%
I only know all this because I was vasectomised 20yrs ago, and recently had prostate reduction surgery (not removal, thankfully).
The other thing to add is that even if you have 'successful' prostate reduction surgery, afterwards ejaculation can 'backfire' into the bladder, leaving you with a somewhat disappointing display of spurty spunk. It basically becomes easier for your ejaculate to go into the bladder than down your urethra. Then the next time you pee, all the diverted spunk comes out then.
This happens to me most of the time, however my orgasms are totally insane and can last a whole minute now. I think this is because fresh nerve endings have now appeared near the surface, who knows? But when you hear women boasting about their juddering, 'whole body orgasms', that's me that is
.