(I've never watched it by the way, never plan to, but I have a vague understanding of what it's about) So my idea is, that some young lady finds out she is heir to a kingdom, like in the film Princes diaries, but the kingdom she ends up in has some erotic twist.
The royal family are considered to represent the wellbeing and fertility of the kingdom. The new princess is expected to so her duty and allow any male of noble birth to try and breed her. Or The princess must wear the kingdoms traditional royal costume, a suit of magical clothes handed down from days of yor. Everyone insists she shouldn't worry if they feel a little drafty. Also she shouldn't worry about the spell on them, they only appear invisible to the weak and foolish. Surely she isn't accusing any member of the royal court of such a thing?
jus primae noctis or The princess is expected to spend a night with every young man who just turned 18. (both ways could be interesting concepts in royal families with strict matriarchic succession, though I guess it's pretty likely that the princess would sooner or later spread STIs)
You could ease into it with the how-to-be-a-princess lessons getting increasingly NSFW leading up to the revelation that she needs to be pregnant within a year. Also you could steal from The Scarlet Pimpernel and have a male rival who falls in love with her and stops trying to start a democratic revolution. ... hope I'm not to late.
