For all those saying that it's insane for her to have been mistaken for a guy by everyone, I disagree. Unless someone is actively looking to see a particular gender, I completely believe that anyone looking would assume she's a guy. Unlike most dramas where a female lead is trying to pass for male, she didn't do the whole awkward "Look at me, dude, man, bro, I'm totally a guy, just... look at my... Um... I can spit far and fart! See? I'm a guy!"
Instead she just kind of lived life. Like, other than binding, she didn't exactly go out of her way to convince anyone of anything. She didn't change her speech, behavior, or anything like that. She held herself in a way which girls get called out for all the time for being too boyish, and she had a haircut that didn't just look like a girl wearing a bad wig. Do I think the ML wasn't straight? Yes, I don't think he would conceivably have fallen for her as a guy otherwise, especially considering he was wholly attracted to her and wanted to go farther while under the impression she was male. Buuut sexuality's a spectrum and I applaud the character for coming to terms with his attraction to a guy in the first place, and for the side characters being supportive and all that jazz.
I have rewatched this multiple times and will likely watch it multiple more times. I genuinely love the acting in this and how for once in a drama the FL doesn't go over the top dramatic in her "I'm Totally a Dude" shtick and instead just continues to act naturally as just a bit of a rough girl. Anyone who says she's obviously a girl is only saying that because they know her, because I guarantee if you saw her looking like that in public you'd either assume she's a guy or at least stop to question her gender. It's only obvious she's female to the viewers who know or know the actress from other works where she presents as more feminine.