Well, at least he didn't need to tell Aizawa that he is rejecting her because he is nearly twice her age. That rejection never works well because the student is always like "I don't care!" as if them not caring is an acceptable response to someone not wanting to date someone half their age. All those responses that are like "you have so much to live for, I am rejecting you for your own sake" are bullshit, because if the kid is serious, they probably already thought about that. In the matters of love, it really is better to give a selfish or personal answer, because kids don't realize that love involves more than sacrificing their own future, youth, etc, and they don't realize that rejection may not be because there's something wrong with them. If they really put that much thought into it, or think they put as much into it, then giving them an answer they don't expect can derail their entire train of thought, and gives them something else to consider in the future. It's a learning experience.
And yeah, he can't understand how in love she is with him. She could be 100% in love with him. That is valid. Some teenagers fall and they really do feel like they will never love anyone as much ever again. But 100% of someone's love at that age is only a tenth of what many people will experience in the future.
I also relate to the sentiment of wanting to learn everything about the world, but not meeting new people, but that is paradoxical and impossible. Let's say that he DID accept her confession. Even if he wasn't such a miserable person, at this rate, she'd be so attached to that she may not be able to realize that she may disagree with his views sometimes. She would never actually learn about the world for herself. She isn't looking for him for guidance, but answers. But she's a smart enough kid. Eventually, she might realize that even he doesn't have all the answers and that he isn't as wise as he had to seem as a teacher (especially if she wants a reciprocating romantic relationship where they tell each other about their feelings and worries and such. Gawd that would be an entirely other discussion of whether she is in love with her perception of him, and what she will feel when she sees him when he's weak), and then what?
And what's so dirty about getting married and then realizing that maybe that wasn't the best idea? Some people have traumatic experiences they, put lightly, need to sort out. Some people better suit working on themselves first. Some people just don't want to get married. Did he say something we didn't get to see after? Was he playing dirty by asking the question beforehand? Cause like... I imagine he would give the same answer regardless