This is not an issue of the woman not being a virgin, but of the husband slandering his wife. In ancient custom, the parents of the bride would keep the bed sheets, and if the husband slandered their daughter, they could take him to the judge after putting some blood on the sheets, and he would be libeled for slander. This law became more important when women were put to death for not being faithful to their husbands, or future husbands.
This type of slander was seen as an attack on the woman’s parents, the slander was not merely saying that she committed adultery, but also that she did not honor her parents, the greater sin in the minds of people at that time. There is no law against premarital sex in the Torah, but some parents sold their daughters to their husbands as virgins. Saying that his wife was unfaithful was an attack on her parents in this manner, as they were being accused of robbing him.
While the plates of brass do not say that the woman should be put to death, the later version of this text found in Deuteronomy 22:13-21 does demand that the woman be put to death if they were not virgins when married. It should be noted that Book of Remembrance 22:12 defines a virgin as “a pure desire to bestow.” While people may have read Deuteronomy literally, this more likely was mean to be understood spiritually and “virgin” should be better defined as a righteous person rather than merely as a woman that has not been with a man sexually.
See 5 Moses 23:13-20, Deuteronomy 22:13-21