Well, to me, they both have their faults, but their feelings each make sense.
For Wendy: She initially only went along with the marriage promise so she wouldn't lose her magic, because as said by best Papa Shakma, if wizards/witch's words hold a lot of power, and if she breaks the promise to be with him, she will lose her magic, which she has always held precious to her. Magic defines a big part of her identity and kind of gave her strength to who she has come to be. It also is a necessity for her job in magical research, with which she makes a living and has devoted her whole life to.
She has asked Aidan multiple times to change the promise, because even if she was catching feels for this man, she wasn't ready to devote her life to him. Her devotion to magic is still very strong, and 10 years(that's a lot to a human) of feelings for another man are still lingering inside.
And after finding out she's getting married to a great, powerful, and mysterious creature known as the Dragon in their world, an IMMORTAL creature known to trick humans, she is going to understandably be
1-- pissed that he decided to leave such a big part of his identity a secret while pushing for this marriage for a whole month, leaving her totally in the dark about his own family and origins/what she would be getting into when marrying him(she doesn't have a dragon sensor and she didn't know the title of this story) and
2, doubtful of whether this marriage is worth going into when he's hidden so much and there's bound to be more secrets. Dragons also have a reputation for manipulation.
She gave him a month. And she had built so much trust and love towards him within that month. Not enough to be totally sure about marriage, but enough to solidify her feelings towards him. It's enough to consider giving up the livelihood she's so devoted to that is such a part of who she is. And although he messed with those feelings, they won't go away. She already developed a strong emotional connection with him that she hesitates to let go of. She is carefully weighing her feelings towards Aidan, marriage and her own aspirations, because she cares for him, and doesn't wanna go into marriage with him half-heartedly. She has tried to ask more about him, but he's still hesitant to share everything to her because he's so afraid she'll run away. He doesn't realize that she's bound in this promise because she doesn't wanna lose her magic and she's emotionally attached to him. Despite feeling betrayed by him, she continues to follow him because she still cares for him and wants to know more. Keeping magic is now just an extra excuse.
Now he's pulled this stunt, forcing immortality and shared emotions upon her without warning and consent(even after she refused earlier) so now she's EXTRA bound to him and can't do anything about it and wouldn't do anything prior cuz she wasn't given a clear definition of what the marriage promise would entail cuz of mr. secrets man Aiden.
For Aiden: What he did was WRONG....BUT, to PUT THINGS IN PERSPECTIVE, it is important to remember that he woke up not too long ago from his hibernation and unexpectedly, his mind had not matured and stayed pretty much the same from when he was a hatchling. His insecurities, his temper, impatience...they're all still deeply rooted in his personality. IN ADDITION, to him, her hatred is a temporary set back to the almost eternity that they will spend together, because he is in the mindset that things shall pass with time, and he is used to having a LOT of time. Again, this isn't really a defense for his actions, but just putting things into perspective. Perhaps, after feeling the magnitude of the internal conflict going on in Wendy, he will realize the consequences of his actions, and try to fix things with that "oh so powerful" dragon magic of his. But not secretly. He will actually communicate with her his plan to amend his mistakes.
I'm actually curious as to what more could this man be hiding. There has to be more cuz he's hidden a lot.
