Cuckolding  is in fact very serious. It comes under the category of ניאוף. [adultry]The definition of adultery in the Torah is sexual intercourse with a married woman or a betrothed woman by anyone other than her husband.   It makes the woman forbidden to her husband. The children are ממזרים bastards, [and can not marry an Israelite woman]. If the act is done in front of two witnesses there is the death penalty for both the adulterer and adulteress. [Leviticus 18 and 20]

Gentiles are entirely unaware of this. They try to piece together  a coherent idea of what the Torah holds based on random readings and scraps from place to place. Clearly that can not work.They do this based on Luther. That was a reaction to the problem that the Catholic church which had started out sincere and  had become a self perpetrating bureaucracy. So on one hand he was doing a good thing- but it left a lot of misunderstandings. Since then it has been up to debate what the simple meaning of any verse is. How the individual wants to understand it? How the author meant? How the people reading it thought it meant when it was written? Etc.