I at least, viewed Yosuke's mistreatment of Kanji's sexuality as him being 17. Not every teenager is Jesus Yu NaruChristmi, many are woefully stupid, rude, or ignorant. Yosuke's behavior toward this part of Kanji was something that made him that much more believable to me because when I was that age I was a somewhat ignorant shit toward gay classmates, too. I grew out of it soon after High School and I kinda expect that Yosuke will, too, because his bad behavior doesn't stem from hatred, but just a youthful dickishness. It's reprehensible, yes, but a deep seated evil it is not. Yosuke's the kind of guy who says dumb shit without thinking, yes, but he's also very aware of how much that sorta tarnishes him and is driven enough to want to overcome what a crappy guy he can be, I think. Which is why I think Yosuke's great, because he isn't, but he wants to be and uses that to better himself. As he gets to know Kanji more and is exposed to Yu's example, I expect it won't be a big deal.
I guess I'm just projecting myself onto him because I see a lot of myself in him, but like, if he is how I see him, the real world will set this ignorance of his to rights.