Onan disobeyed Gods direct command. Genesis 38: 8-10.
Onan has absolutely *nothing* to do with birth control or premarital sex or even masturbation. It has everything to do with essentially killing his brother’s line by not giving him children—basically akin to murder.
Onan’s failure to give his brother’s wife children is almost akin to Cain’s killing Abel. That’s why he was struck down.
It has absolutely nothing to do with the supposed sacredness of male ejecta.