Will probably be explained in twenty to thirty chapters but here's a brief explanation on quirks of royal family. The royal family are descended from dragons with an enormous OP amount of magic power appearing in men, (not sure how affects daughters or if all children are always sons.) Problem is that it slowly drives them crazy and is hard to control as they get older due to intensifying libido. Hence they need to find a wife to mark that'll stabilize them. Mark appears as a flower when he applies the spell while taking their virginity. Guys only get one chance to mark a woman and she HAS to be a virgin. He can get all the experience he wants but other than his chosen (or someone thinks is his chosen,) it'll be unpleasant, uninteresting and mostly unsatisfying sex for him. Basically only his chosen is his type and all other women really aren't his type. If woman not a virgin or something goes wrong and the mark doesn't stick the unlucky man will never be able to fully control his power and be essentially exiled to decrease risk to the people and will never become king. Reason for polygamy is due to the dragon blood their libido is increased and are trying to avoid wearing out or killing their chosen one by snu snu. Basically he'll be able to go all night every night without getting tired due to his lineage while wife just a normal human who'll have trouble keeping up. Then there's everybody who thinks only having sex under strict rules according to the stupid book of etiquette is proper.
