You and I are seeing this differently. If someone is head over heels in love with you, the least you could do is tell that person to their face and not tell them "I see you as a woman now." That just makes it worse. In some cases things just needed to be said clearly, esp since Yona so stubbornly loved him.
I don’t think that Su-won knew that her crush was such a serious thing or expected that it was so. Well, in a way it is impossible to say how strong someone’s feelings are unless you can do mind reading. Even if he knew about her crush on him, he might have thought that it is nothing serious or just one of her whims.
Besides, when he asked why she came to her father’s room and he why she was awake so late at night and got his answers that she wanted to tell father that she couldn’t forget Su-won, he is shown with eyes wide open, so I always thought it was an expression of surprise.
http://www.titaniascans.com/reader/Akatsuki+no+Yona/002:+A+Torn+Bond/#page=16
We know that Yona was very serious, because we have access to her thoughts as readers and we know that she wanted to marry him. Did Su-won even know that Yona wanted to marry him? It doesn’t seem so, so it very likely looked different from his perspective.
Yes but I am not only speaking about this. I am speaking about all his feelings about Yu-Hon, Il etc. He never shared that with his supposed two closest friends and always had a large wall around him.
I think if Hak asked Su-won about his reasons for which he cannot marry Yona, Su-won’s answer would lead him to it. Even if Su-won didn't tell him, he would likely get more hints, put pieces together and discover it himself.