Basically they both had some interest in eachother prior to all events but for status and political reasons should not be together. (Putting aside his race is basically a sex demon that slowly changes their partner's body to make conception easier).
FL had an argument with her father about taking her servant (that everyone thinks she was sleeping with but wasn't) with her to wherever she goes after getting married off to a rando. That same night she ran away to her servant's house (definitely proving them right) and when he was tucking her in and getting tempted enough by his own desires that he was planning to kiss her and probably not stop there; she ended up jumping him believing to be dreaming herself. Next morning, ML kidnaps FL and rapes her repeatedly in the hopes that she will want nothing but him (even though she already did but couldn't publicly because of her duties as a princess/Magi) creating a miserable situation that could have been solved by surprise surprise communication.
Any questions?
I don't understand the plot and feel like I lost some brain cells in the attempt T-T