ML's rejection for her was because he doesn't want her to waste any more time on him, because as "someone who also has an unrequited love" he thought he understood her unrequited love towards him.

It was indeed a deliberate set up by the Crown Prince, whose initially thought was to make the two become more acquainted to each other because the union between the two of them is highly beneficial for him. Well, he only set it up for that, he himself certainly didnt expect that the two would ended up very physical.

Which includes proposing to her, after he publicly rejected her marriage proposal literally a month before. Sadly, coinciding with the Crown Prince's plans who wanted their union, that's where the misunderstanding arises. She thought she's being used in this and wanted him to clear it up but, he just started liking her and can't confidently say he loves her yet but he indeed wants to marry her.

Bro you rejected her at first because u liked someone else but now your crush is marrying someone else and youre all sad and all but your cousin was like "hey if u marry her it will be beneficial for me" but uhhh "awkward cause I just rejected her" but your cousin wouldn't give up and was like, "its fine how about I'll bring you two and some other nobles so it wouldn't be obvious it's just you two, for some retreat and yall can get to know each other" and now after 'getting know her very well' you felt like... Hmm maybe marrying her probably won't be that bad but then she found out it was kinda a set up (although imo it's not really one because the crown prince cousin wasn't really forcing them lol, he legit just wanted them to know each other better) and now she's mad and asking "do you even love me to marry me" and bro was like..."uhhh I do like you tho and there's no one else suits u more than me"..... bro... he's not toxic of red flag but he's a bit too honest and bad with his words😔