I think it's kinda implied after the mc got drunk. There's a lot of close up shots of his masculine features like his hands and his adam's apple and shoulders and stuff. So the story subtly implies that Yoonsung is seeing him as a guy now.
SPOILERS
ml knew mc was a man and didn't care but he pretended not to know, since if the mc knew he was exposed he will use it, the fact he is a man, as a reason to refuse the kindness and to build a gap between them
Awwwww I was hoping this was the case because both MC and ML are so precious
Wait I read this a while back bur I wanted to make sure of something, he never knew MC was a guy not a girl prior to his memory loss, right? I'm hoping that's not the case and that he knew and didn't care because if he didn't then he will feel betrayed time and time again after regaining his memory. That would be awful to read