Oh yeah let's just treat our FMC like an object with no opinions of her own, no ability to speak out or act on her own. She's a thing to be fought over as the plot demands because this story is dogshit
Did you or did you not notice how she is inexplicably unconscious?
It's very likely she did tell him "Thanks, but no thanks" and he decided to take the fait accompli route by knocking her out so no one else who isn't under the Prince's thumb would know she said "No", and then goading her "knight" into an "honorable duel" by having him "fight fairly".
Also, Nadja is not a fighter. She's a pacifist. She wants to live a quiet life, and peacefully raise a family with the man she loves. But she knows very well that pacifism only gets you so far because she knows the story of her family. She knows that there will be people who come for her just because she is a dragon. We have already seen this with the First Prince.
This is why Nadja has her "knight" - someone who will act on her behalf when she is unable to or someone pushes the matter into outright violence.