To be formal is to make a decision about which side of the dichotomy you wish to be, and stick with it for the rest of the night. There is nothing that translates or parses differently if you change your posture, how tight the belt is, whether or not the shirt is tucked in, all the little differences in coding that you can accomplish with a t-shirt and jeans.
I just don't like having to pick a gender, especially if there's not something (internal, or more oft, external) driving me to identify along the binary. Most of the time, I get away with just being a person, small, silly, slightly scruffy, and essentially neuter. I can do that in my everyday clothes, it's easy to just be viewed as another warm body on the train. But formal, real, proper clothing comes in precisely two forms: stuff boys wear and stuff girls wear. There is nothing occupied by both.
This is a large part of what has been making formal balls suck so hard for me these last few years. Because there's always been something there, I haven't attended a single Ball that wasn't preceded by anxiety about my clothing. Whether I am choosing to be a boy or choosing to be a girl, I have to choose. When my gender is so constantly in flux, when I can go from definitely a boy to definitely a girl in the course of an hour, when I don't ever -ever- know in advance what name I'll want to use upon waking up...To be frank, trying to book my gender in advance is impossible. It'll just lead me to feeling constrained, trapped.
The dysphoria as I switch pronouns in a pair of jeans is managable --at least I can hold my posture different and pretend I'm recognizable. Wearing a full cocktail dress, or tails and a top hat? Oh no. There's no easy way to break free of social cues that strong. I pull on a gender when I get dressed for the ball, and believe me when I say I'd rather just stay indifferent to the whole thing. But formally, alas, that simply isn't an option.