@trapsarebetter Regarding your Sazanami Cherry example, I'm not sure that is a good comparison due to how much trans-coding is in that manga. Ren often talks about how she feels at ease when perceived as a girl, and how scared she is that she'll have to eventually go through puberty. Ren does refer to herself as a boy a couple times, and at least the first time its played more as a joke, but later in the story the context is one of self loathing, e.g. saying "If you're okay with someone like me even though I'm a boy, I can only think of you as an idiot" in a scene where she's crying and trying to give up dressing as a girl. The implication is that she's thinks she's somehow broken for being born a boy. And the ending is all about Ren accepting her own feelings and living as a girl full time, like her sister. Granted no one explicitly says the word trans, so there is a little ambiguity, but the text heavily suggests it imo.
In contrast, I think Prunus Girl is far more comparable to Otome Danshi, since Kizuna regularly asserts that he's a crossdressing guy in a very confident way, just like Yuuki. But your concerns about whether a usually straight guy can love a cute boy don't hold water there either IMO. No one stays cute forever, independent of gender. Age is inescapable for all human beings. Sure male puberty can make it harder to look cute, but it can be countered; irl crossdressers have been using makup, wigs, etc to counteract puberty for hundreds of years. Also these guys are like 16 - they've likely already gone through a significant amount of puberty. And if the last chapter of Prunus Girl is anything to go on, Kizuna will still look pretty as an adult, even if he's a bit taller.
My point here is that if someone falls in love with a cute boy, then as long as they love him as a cute boy and not a cute girl, they'll probably be able to continue to love him as he gets older, even if neither partner is as attractive as when they were younger.