In case anyone missed it, when you look at the reflection in Yoonsung's eyes you can see the silhouette of a man standing before him, not a woman! Another hint the author dropped that Yoonsung already knows the Hakyung is a man. As well as Yoonsung not wanting the younger girl to sleep in the same room as Hakyung, because it'd be improper!
In case anyone missed it, when you look at the reflection in Yoonsung's eyes you can see the silhouette of a man standing before him, not a woman! Another hint the author dropped that Yoonsung already knows the Hakyung is a man. As well as Yoonsung not wanting the younger girl to sleep in the same room as Hakyung, because it'd be improper!