i bet sangwoo would love to debate with you
That's the thing, the things Sangwoo does are only really rational to him, he makes an internal framework for the world around him so to make it easy to navigate. And we've already witnessed that in the first chapters of this series when he had those extremely specific routines because that's what helped him organize and live his life.
At first glance his thoughts and processes may seem rational, and yeah maybe if they're only for him they could be, but soon we discover that as Sangwoo branches out into more of the "real world" (different relationships with people, different processes of handling things) his frameworks won't work very well anymore. Simply put he can't apply these overly simplistic, black-and-white ways of thinking when he's surrounded by such a complex world and the unexplored realities of his own mind. It's almost like wishful thinking in a way. At the end of the day, he's not an AI or a computer so he's going to be flawed like all of us, and that may cause him trouble, and give him "errors", but I just adore having a character that tries to be so perfect, and is convinced of that, realize that not everything can be so impeccable and down to a T, and grow to understand that that is totally okay.
Considering he's so "rational", he does and says a lot of irrational things. His routine, down to the foood he eats and paths he walks are all irrational and based on personal preferences. Hobbies in general is a counter argument to the whole "for the society" POV. Not to mention, biologically, many species engage in same sex behaviours. There are insects, birds like penguins, fish, and lizards that actually mate with the same sex. Left over male penguins even bond for life and raise abandoned eggs/penguins together.
There's a whole thing behind it. Gay, trans, non-binary and intersex people have also existed from the beginning of time and recorded as far as 60,000 years ago in some of the oldest cultures in the world.