Not even excommunicated nuns are that easy.
Well, he did come to her rescue when she was outnumbered, and nearly die shielding her from a sneak attack, before she even knew his name. And he didn't seem to have any ulterior motive, since he was just going to leave after she finished bandaging him. I feel like people are underestimating the impression that would leave. That's legit fairytale territory. And as a knight, she probably has a sense of honor that would compel her to pay him back for possibly saving her life. Not necessarily by
marrying him, but from what we've seen of her attitude, she's not very approachable, so a total stranger nearly dying for her probably lowered her defenses in a way she isn't accustomed to, and a few compliments from a handsome guy she doesn't detest had quite the impact. Also, she's on her own in a city where someone wants her dead, so a partner she can trust to watch her back is bound to appeal to her, even if only on a subconscious level.