nvm I found it:
This only speaks towards her having to report if he won’t marry her.
Duty of brother in law = marriage
Onan fulfilled that duty. There was nothing for her to report by the law she probably knew nothing about anyways.
Let’s broad the Bible citation a bit.
If brothers are living together and one of them dies without a son, his widow must not marry outside the family. Her husband’s brother shall take her and marry her and fulfill the duty of a brother-in-law to her.
The first son she bears shall carry on the name of the dead brother so that his name will not be blotted out from Israel.
However, if a man does not want to marry his brother’s wife, she shall go to the elders at the town gate and say, “My husband’s brother refuses to carry on his brother’s name in Israel. He will not fulfill the duty of a brother-in-law to me.”
Then the elders of his town shall summon him and talk to him. If he persists in saying, "I do not want to marry her,"his brother’s widow shall go up to him in the presence of the elders, take off one of his sandals, spit in his face and say, “This is what is done to the man who will not build up his brother’s family line.”
That man’s line shall be known in Israel as The Family of the Unsandaled.*
Deuteronomy 25:5- 25:10
As you can see, the whole point why a man must marry his deceased brother’s wife is continuation of family line. Something Jews of the OT cared a lot about (I guess much more then “Natural law”). The firstborn would be counted as the son of deceased person, and the line would continue.
If deceased brother had a heir, marriage would not be needed.
Since there was no sexual life without marriage, a man had to marry a woman in order to fulfill his duty towards his brother. Marriage is just a step in system of keeping the bloodline flowing.
Duty of brother-in-law: to marry, and to give heir to his brother.
If one marries a woman but does something in order to prevent levirate duty, he preformed sin. This seems to be Onan’s case.