@givemersspls I think the situation is slightly more nuanced than you're giving it credit for. When Sacchan asks her brother "If I were to say dating Mogumo would mean that you're gay, would you just give up?", it is in the context of Tetsu struggling with the idea of being queer. Over the last few chapters Tetsu has been examining the world around him and how it treats queer people, and among his revelations of how poorly queer people get treated both in real life and in fiction. And on top of that, he is starting to examine his own sexuality and is worried that he will be treated similarly poorly to his friends.
When Sacchan asks Tetsu that question, it is to affirm that he shouldn't let others perceptions prevent him from pursuing the person he loves. Tetsu won't give up on Mogumo just because others will see him as gay.
The question of "is it gay to date a non-binary person, regardless of how they present?" is a complicated real issue that exists, and really, the only real answer is an incredibly gay shrug.