As noted:
Onan’s sin was not in masturbating (or even coitus interruptus, per se), but in enjoying sex with his brother’s widow, purportedly to follow levirate law and provide his brother with an heir, but then preventing conception, so that his brother’s widow would have no children to support her in her old age while his dead brother would have no one to carry on his name and memory. It was not simply “spilling seed” but a form of rape.
There have been a few writers in the Christian tradition that made the same mistake you have and assumed that Onan’s sin was the specific act of “spilling his seed” without any context. They were wrong.
Along with a misunderstanding of the actual sin narrated regarding Onan, the same people made a separate error in that, lacking current biological knowledge regarding meiosis and DNA and relevant subjects, they assumed that the human male created a “seed” that contained all that was necessary to form a new human and that the woman’s body was simply “earth” into which the “seed” was planted. Since their version of procreation was based on an error, their version of the sin never included any action on the part of the woman, so there was no “sin” in menstruation.
And, as has been pointed out, no mainstream religious tradition has ever held that natural functions that are not conscious decisions can be sins.