It looks heterosexual boy + woman, and it works 100% as such. Heck, it'd even look believable as an oneesama/senpai yuri crush... but the text says otherwise, for no apparent purpose, it's either BL or a bromance between a het boy and a more mature transvestite?? These premises don't even really work, because characters never seem to give even a moment's thought to neither their crushes' gender - yet also never mention or think of orientation/inconvenience/whatever when hit on by females.
GIVEN (chars, according to the way they're drawn):
A) one androgynous child lead (lacks gender characteristics),
B) one strong'n'violent and flat-chested but super-feminine flirty female lead, and
C) one big-boobed loud weird girl, supporting
FURTHERMORE:
A) actively avoids sexual advances from both genders; professes heterosexuality & male identity to B, yet makes moves on B;
B) actively flirts with everything that moves, treated like a girl... also gets hit on by nearly every adult male out there
C) looks like a girl, acts like a girl, treated like a girl... even scanlated as a girl for the first dozen chs, then suddenly gender-flipped. disinterested in sex, doesn't flirt... AND those boobs, they're clearly there!!
AND!: 20 chapters in, nobody has done the dirty or had any introspection around confusing desires or shifts in gender/orientation identity. nor professed any surprise at sexual advances from an "unexpected" direction from random episodic characters, either, not even giving it any thought
YET!! TEXT SAYS: ...they're 3 males - huuuh 😕 Really, why 😕
Even the genre and category lists here seem to show reader's confusion. And online reader sites, too, call it "Josei" (...is it?) sometimes, but never (BL/yaoi... it is?!?!)
READS like scifi with a background theme of romantic love starting to bloom between barely-pubescent male kid and an adult female that takes him under her wing, initially to repay a "debt" of a rescue when both were still kids... translator could've swapped it as such and not one reader would've ever thought otherwise. It's 100% het-looking stuff that makes perfect sense as het, but is for some reason claimed to be shounen-ai..
...heck, it COULD even be more feasibly rebranded into yuri (even the confusion of the younger protagonist over just about the only clearly-female&identified-as-such sudden request for sex before study abroad parts them would suddenly seem more realistic)
MANGAKA-SAN 😕 ? Were you:
a) Accidentally picked up for serialization by the "wrong" mag, and decided oh-well-whatever?
b) Bullied by an editor to make a BL, and in rebellion decided to just write & draw it as het and then just gender-flip a couple people?
c) Utterly unable to write anything but het, but really wanting to cash in on a fad, and deciding to just masquerade a het manga as homoerotica yet not actually bothering to do it??
d) OR - so used to writing BL that you subconsciously gender-flipped a few characters, not realizing that the story you were writing was het in your head AND worked better as het??