I've come across a significant number of people online who genuinely didn't know homosexuality existed and didn't realise they were gay for years because they didn't think it was even a possibility.
Point taken, but there is always the question of their age. If they originated in the bury-your-gays period, then yes, I can imagine them, even if they use the internet. I know people of older generations that can barely use the Word, let alone browse intensely through the net (or having the inclination to do so on a more regular basis). If they are adolescents in the second decade of the 21st century, and are, in all probability, bombarded with popular culture references, and those often do include either speculations, or outings, as to which celebrity could be, or is, gay or bi, than I find it to be somewhat improbable. Also, with many countries engaging in public debates, referendums, and legislative reforms concerning the question of gay rights, with even Japan having made some minor improvements (on a purely limited local level, if memory serves), I would find it even more unlikely that they would be, apparently, completely oblivious to the entire thing. That is just my opinion, though.
As far as I'm aware Japan is notoriously 'don't ask, don't tell', so children/adolescents may very well not see any examples of gay people with whom they may identify with enough to ever think they're gay. Plus, authors usually write from what they know and I presume the mangaka is at least ten years older than Yuma and Hotaru, so her personal experience of being gay (If she is. I'm always curious how many yuri authors are queer.) or awareness of homosexuality as a teenager would be at least ten years out of date.
Also, convenient drama, obvs.
I'm actually really interested in the state of the gay in Japan, but I've found it pretty difficult to find much about it online. I guess if I could read moon it would be easier.