You seem to be forgetting that a LOT of nobility kept mistresses. Sometimes (just one, and sometimes many) for several years, if not for a whole lifetime! So yes...he is very valid in his reasoning that he is currently a mistress!
Pls Alexa is just dumb-dumb, though I cant fully blame him as Maetel is so sly
He literally told him that he was basically a stand in in one of the first chapters. He's also never clarified that he's like him since they were in school together.
Princes: Kisses the MC, Hugs the MC, Sleeps with the MC, search through all the merchants just to get strawberry cause MC is craving it, Gives mutiple hints that he likes MC, feeds MC food.
MC: I'm just a plaything for him...
Me: (╯°Д °)╯╧╧Bruhhh (insert technoblades voice) I wanna hit you on the head