if she got pregnant wouldn't this mean all the peasant's children could have been related to the lords, therefore they were all related across social classes? it was also probably a convenient way to deflect blame to someone higher, if a peasant woman fucked some other peasant men before marriage and she got pregnant or wasn't a virgin they would say "it was that bastard archduke!" and then they might rebel or emigrate to another lord's land to work instead of blaming the woman, or be protective of their same statue women in case she was lured on by superiors. there are many reasons it would be a popular belief and will have happened, but feudalism was thousands of different lands with their own rules. you believe medieval were so different to life but also straightforward, it's literally the chad vs incel meme.   how would you know if it was happening across the holy roman empire, italy, russia, spanish empire, ottoman empire, even amongst muslims. it probably still happens amongst kurds and other feudal people today. there is no document that said it was a law anywhere, you only know about it from us because some monks wrote it down in a story who themselves were volcels. a lord was also a knight, they had to fight to earn their right to own land in a kingdom, how could you compete as a pig farmer? it is also against christianity so it would depend how religious they were.
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