To clarify why, it’s because sperm travels through those tubes for at least 5 days before reaching maturity and then once in position can be ejaculated by a pressure differential caused by the prostate (that bulb thing between the balls and urethra). So an effective switch with an instant effect would need to be placed before but directly adjacent to the prostate which could have potential complications when the prostate attempts to dry fire with the feed shuttered.
The solution is clearly to add a synthetic pressure bladder. (Don’t, I’m joking).