To understand what may be going on here, I think it would be useful to see some pictures of the relevant anatomy. This picture shows this from a urinary perspective, i.e. it omits the reproductive bits, but is a good starting point:
View attachment 77569001
The things to here are the location of the prostate gland and the pair of sphincters both immediately upstream and downstream of it. The upstream one, just at the point where the bladder neck becomes the urethra, and called the internal urethral sphincter, is involuntary, i.e. you cannot consciously control it. It is normally closed but opens automatically when you need to pee.
The downstream one, called the external urethral sphincter can be conciously controlled. It is this you use if you need to pee but want to delay it because you aren't somewhere suitable.
Now to add in the reproductive function, look at this:
View attachment 77569181
Sperm make their way from the end of the epididymis up the vas defrens. As far as I know that isn't limited to ejaculation but happens all the time, which is why you need time after a vasectomy to be clear of sperm because, at the time of the op, some are already up inside the body ready for ejaculation.
When we ejaculate, the sperm are joined by fluid from the seminal vesicles, then that mixture is joined by yet more fluid from the prostate gland. All of this ends up in the bit of the urethra that is inside the prostate gland, between the two sphincters I already mentioned. Assembling the semen there is called the emission phase of ejaculation. You can actually feel that happening before anything spurts out and it feels good.
At that point the internal sphincter will be closed*, to direct the semen outwards and not into the bladder, and the external one will be open to allow it out. Finally muscular contractions push the semen down the urethra and out of the penis.
Pressing on your taint while cumming will have had the effect of blocking the urethra downstream of the external sphincter, much as you can cut off the flow in a garden hose by pinching it. The muscular contractions still happen to a piece of the urethra now closed at both ends: by the internal sphincter at the upstream end and the blockage from the pressure on your taint at the downstream end, so the pressure inside will increase. Something has got to give and what usually happens is that the internal sphincter leaks and the semen ends up in the bladder. That is the retrograde ejaculation referred to earlier.
What is not clear is why that happening as a one off should cause things to malfunction thereafter. Because the internal sphincter is involuntary, I can't see how you could strain the muscle involved. But is there another way it could be damaged so that subsequent ejaculations are also retrograde and end up in the bladder? I don't know? I am not a urologist. I know some people do this thing of pressing on the taint regularly and seem to get away with it, others warn against it. It does seem hard to believe that the whole load would go upstream through a slightly damaged or malfunctioning internal sphincter when the downstream route is clear, unless that sphincter is wide open.
As for a physical cause, there is no point in considering anything upstream of the seminal vesicles as those and the prostate gland make up most of the volume.
* The ejaculation reflex taking over the internal sphincter is why we can't pee when highly aroused and close to cumming. It can also take a few seconds to return to normal afterwards.
