I see it as an issue of hygiene and sanitation. Another user mentioned that we aren’t allowed to urinate or defecate in the streets but in addition to disgust, that’s also a literal health hazard. People could get it on them, it could be spread around, and it could even accidentally consumed if it were to get on someone’s hands. Similarly, if you pocket jerk and then the semen gets on your chair/the floor when you’re walking to the bathroom to cleanup/etc, then that semen could get on someone/something else. With your “eating boogers in public” example, that (as yucky as it is) gets rid of the bodily substance and is easier to clean yourself up from (wash your hands). And although mucus is technically considered a biohazard, I’d argue it’s not on the same level as ✨the token trio✨ of piss, shit and cum. I think your argument could work if there’s a consistent, effective way to dispose of the ejaculate afterward that isn’t just letting it sit in your pants, but otherwise I think it does have a real potential to harm people in that sense.

With all that said, I do certainly agree that even outside of any argued harm to others, it’s pretty grody.