His brother, as head of the family sold Conde to Elizabeth.
As far as Elizabeth is concerned that is her finance, and she's probably told a lot of people who will laugh at her (behind her back) if that is not her bloke after making claims that he was. Cheating on Lizzy or ditching the Queen of England is a capital crime that results in a beheading.
Even the failure to/attempted marriage to Elizabeth will be seen as treason to the French, and be an act of war, if it's even rumoured that English soldiers will garrison in the little principality Conde comes from, which obviously is going to be crushed, burned and salted by Franics before it becomes Elizabeth's new foothold on (in?) France.
What Conde has done is not betraying his girlfriends affections, but agreeing to be an English Flag in a political game that's bound to be a war any day now. If he stays in French court, Francis will cut his head off for being an English Spy/Agent/Subversive.
If on the other hand Conde goes to Scotland, then England and France will both try to destroy him for stealing one queen and denying another, which is not ideal. Not ideal is better than the impossibility of staying in France once he is labelled a criminal and a traitor.
After he marries Elizabeth he is safe, a Prince of England even, before he says I do, he's just some #### who can be pushed down a well and no one gives a ####.
It's not Mary's hurt feelings that are the problem, it's Mary's mouth. If she repeats the story once by accident, Conde's limbs are ripped off by wild horses that's one thing, but if she repeats his truth to exactly the people who will sign his death warrant because she wants to see him ripped apart by wild horses, well that's just to be expected from hell's fury.
Mary is 15.
Stop thinking of her as an adult that can see more than 5 seconds ahead of whatever is in front of her face.