No clue how this became such a big taboo in Abrahamic religion,

Iirc it's about not "wasting" life essence.

Genesis 38:9-10: But Onan knew that the child would not be his; so whenever he slept with his brother’s wife, he spilled his semen on the ground to keep from providing offspring for his brother. What he did was wicked in the Lord’s sight; so the Lord put him to death also.

It's that Monty Python Every Sperm is Sacred song, but unironically

I had an idea for a rpg once about a near-future society where jury duty lasts like five years. Like they lock you in a bubble village and cut off all your access to the outside world so that when you go to sit on a trial you have absolutely no foreknowledge of what it's about.