@nobile Telling her would have meant telling everyone if it had happened early on. She's a professor, or something along those lines, so if she knew there's a dragon posing as a human in the academy, she would have definitely reported it to relevant authorities. Now, I have to backpedal a little bit because of course it's possible those higher authorities would have wanted to keep it a secret, but it would have still ruined any prospects of a romance, no doubt, for numerous reasons.
Without that initial promise, I feel like he would have failed miserably even if he was just another human being. As soon as she sobered up after the first night, she began to reject him. He had to use his best efforts and wiles to gouge a promise out of her. Even after that it wasn't easy to win her over. So, in the beginning it didn't much matter if he was a human or not. With the par set that high, once again, revealing he's a dragon would have ruined everything. I'd say the dude could judge that by interacting with her. Now, after all that work invested in charming her and genuinely getting her to agree to the marriage, she still almost dropped him like a hot potato after hearing the truth. Sure, she feels betrayed, but it couldn't be helped. Maybe she realised it herself.