Curate, connect, and discover
from the transgender reading perspective, the thing about yashiro's assumption that his stepfather hated the idea of his body becoming "male" which makes the emphasis so important, is that it's all it is - an assumption. not necessarily a wrong one, but one made three years later, specifically after (and because of?) experiencing a form of rejection by kageyama that yashiro ties to his bisexuality and, by extension, gender as well.
it's easy to read projection into it, at least to a degree. the feeling of wrongness in your own body, constant and unrelenting, that yashiro never puts into words. dysphoria, if i had to put it into one. because it's not just about becoming an adult, it's about becoming "a man", about puberty and changes that may have been unwanted. "he must have hated it, and part of me hates it too".
and that's the pattern. everything always, always circles back to "being a man" or rather - "not a (cis) woman". disgust, hatred, abuse, desirability as a sex object, and only as a sex object but never a person, are all easily explained and justified by that alone. yashiro thinks that kageyama stops touching him not just because he's bi, but because he's "a man who likes men". it's a wrong assumption, which is why kuga hurt as much as he did, but it's important that that's where yashiro's mind goes to and that it stays with him for years. how his stepfather actually felt doesn't matter, but if his hatred towards the changes in yashiro's body played any role in him leaving, then it's ultimately a good thing. still, though, that's where the mind goes. the connection between "maleness" and aversion, even when it may be desired, is always there. and it says more about yashiro's feelings than it does about anyone else's.