That being said, I won't disagree that morinaga's characters could try and google "homosexuality in the 21st century" once in a while
That seems to be true for lots of characters in lots of yuri, one-shot and otherwise.
I mean, look at Yamada in Kase-san. Not only does she seem to be completely oblivious to girl-on-girl sex at the age of 17, she seems to have never been exposed to any kind of sexuality whatsoever. You get the feeling that she has never been sexually aroused in her life, and that she has somehow managed to totally avoid any and all depictions of sexuality in media for 17 years. And yet Kase-san is widely loved. (I think it helps that Kase-san herself is just straight up and unapologetically gay as shit.)
She's not the only one, but she's the one who comes to mind first for me. I've only been reading yuri since October, but I have already seen the "but we're both girls!" trope a few dozen times already.
As long as the cliches are done well, I don't mind them. Just like I occasionally want a cheeseburger despite there being absolutely nothing innovative or revolutionary about a cheeseburger. And I think Morinaga does these cliches well. I really do feel like Hana and Hina are in love with each other, because of all the details Morinaga puts in to sell it.
I mean, contrast this with Out of the Blue! Now, OotB! is one of my favorite stories here because of its emotional resolution in chapter 3. I really do like it a great deal. But before the emotional resolution in chapter 3, I wasn't sold on the notion that Risa really loved Kazumi. It was all tell, no show. Risa loves Kazumi. How do we know? Because Risa says so. And I'm not at all sold on the idea that Kazumi loves Risa. Not because I disbelieve the author when she puts those words in Kazumi's mouth, but again it's all tell, no show.
Morinaga sells it. And that's why I like this series so much.