Heh

The book of period of time is more akin to the middle times in Europe. This implies that in mediaeval Europe, feudal lords had the legal authority to engage in sexual relations with any servants that within their reach or find appealing. Monarchs may do anything they pleased. It was an immoral action to do and especially law and morals were subordinated to royalty and the church. So if you're a commoner or a servant your opinion and stand doesn't matter.

To make the story more practical to the closed to the era, the author portrait the top to do as it please and be also arrogant because his the duke everything he wants he can have it. Also there was no consent required for the royals in this era so they can do as they pleased.

Honestly the plot is good if we ignore the fact that the royal forced himself to the mc, just saying y'know? it's not common now a days to have plot like this in certain bl's and they are portrait into fiction but also closes to really in certain based situation. If this has novels or raws maybe the duke would probably go under some character development changes the story is 10/10 tho personally in my opinion.