There wasn’t one moment. You could tell throughout by Seungho’s actions more than his words. He just started being more talkative and that’s only fever talk. On the other hand NaKyum is way far behind, he isn’t in love, he did though find someone to accept him, whereas others mock him for his origins, his work, or berate him for his sexual orientation like his teacher did. NaKyum only started admitting he enjoys the sex in the last couple of chapters.
Lol did you really read the story? SeungHo has a soft spot for Nakyum from the beginning. He is violent and problematic to everyone around him, but in the end he would always give in to Nakyum. As if there would be a boundary of an sudden event where they would fall in love out of nowhere as you are suggesting. It was very well built. And also you are mistaken. They did not fall in love, SeungHo did.
I think Seungho started to like Nakyum when he realized that Nakyum wasn't easily swayed by his nobility. Think about it. All the people surrounding Seungho are either servants or other noble men just using him, so realizing that Nakyum didn't want to be with him even after having sex with him probably peaked an interest. Also seeing how passionate Nakyum was when he mistook Seungho for Inhun made Seungho yearn for that type of affection.
So can anyone tell me at which point did they fall in love ‘cause I feel like they just suddenly fell in love for no reason???