I understand why y'all would think he knows already. The FL herself thought he discovered the truth when he walked in on her bathing. However, despite being given high intelligence stats, it doesn't overwrite the fictional character curse that makes him oblivious to all things revolving around love. Also, he was too busy in the process catching feelings to be catching a true glance of his beloved's body. He processed that he glimpsed at his crush nude and his senses went into flustering overdrive blinded by the heat rising in all the special places. Female lead just did what female leads do and jumped to conclusions without talking anything out. Thus the gender-reveal party was canceled.

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If anyone is worried they kiss in chapter 42 (he also still thinks she's a guy lol) and he doesn't find out she's a girl till 51
When I say he I mean edward