What is the logic behind the prohibition of ejaculating for no reason?

It is very likely that the Sages understood this from a sabbath, and assumed that Onan was also supposed to understand this. It is unlikely that this is because of the harm that this brings, since the commentators attribute it to adultery by hand or to the accessories of the darziha. No one explains this from the law and the preservation of your souls or the preservation of your health. Cain was also supposed to understand that it is forbidden to murder before there was a command. The assumption is that prohibitions on certain acts are self-evident. The source cited for this is not always the real source. Sometimes it is a vague reference. There is no one among the enumerators of the commandments who specifically enumerated this, and therefore it is likely that they did not see this as a prohibition with a clear source. Beyond that, even if Onan was not supposed to know this, the Torah's description of the matter is the source, even if Onan himself is not. Just as there are those who learn not to marry the younger sister before the older one because they did not do so in Laban's place. And is Laban a halakhic source? The Torah's description gave them the understanding that the Torah itself sees this as a correct principle.