No one really knows for sure where sexual preferences come from.
We don't have a rigorous model that explains every nuance; sexual preference is a very complicated topic, but we're far from knowing nothing about where they come from. We know that postnatal hormones aren't determinants. We know that gonads, including the removal or alteration of them, aren't determinants. We have strong evidence to correlate sexual orientation with prenatal conditions, and brain structure. And we have compelling models to link those correlations to causative functions.
To simplify, we have a pretty decent crude understanding of the broad mechanisms. We're just still working out some of the fine details.
As I said Rinko may be bisexual and the change made it more obvious, with erections, whereas it was maybe more discreet before.
This could partially explain it, definitely I agree, but If this were the only explanation, it would not explain why she has no reaction to men. This would heavily suggest that sexuality is intrinsically linked to one's sex organs (and again, this isn't a rare thing for GB manga to imply), and we
know this isn't the case. That should be pretty obvious and intuitive if you believe that a masculine man can be gay, for example.
This is why I offered the closeted lesbian explanation in my first comment. Maybe, as you said previously, her crush on Yuuki was just comphet, and she's actually always just been into women. It's just that now that she has a penis, she's no longer able to sweep her sexual attraction under the rug, and it's more obvious that what she feels towards Yuuki isn't sexual attraction.
This explanation works better for me, and the comment about liking boobs works with this interpretation, but Rinko's inner monologue in ch. 2 pg. 25 about her heart being excited but not her penis can kind of go either way and feels suggestive of her genitals being in control of her desire. Rinko also walks back on the idea of her being attracted to boobs, saying it was more of an admiration because she had small boobs herself. This could either be her being closeted, or a legitimate explanation for what was meant to be a gag line.
If it was just these things, I could accept it at face value, even if the manga decided not to explore it further at all.
So it's not out of this world that it may entail changes in the brain that would incur a shift in preference. Since it's not a real disorder, anything goes.
If it was just kept vague and fuzzy I could buy this explanation. Even if the brochure said, "You may experience changes in sexual preference due to the sex change disorder" and kept it simple as that, that would be fine.
But the brochure (or if we're staying charitable, Rinko's interpretation of it) said, "Due to
hormonal imbalance... there may be excessive sexual arousal towards...
women's bodies"
It kind of goes out of its way to link sexual orientation to hormones and thus gonads.
Rinko can't get it up for Yuuki. Is it because she's not actually sexually attracted to men? Did the sex change disorder alter her brain? Maybe either of those, but the manga has already kind of primed us to link sexual desire to hormones and genitals. Following that logic, Rinko isn't physically attracted to Yuki because she now lacks the equipment that produces attraction to men.
Is Rinko getting excited over her female classmates because she's always liked boobs? Maybe. Rinko claims she didn't like them this much before, and her friends come to the conclusion that it's the reaction of a boy. They link the sexuality to the gender, the sexuality has already been linked to the genetalia, now the genetalia has been linked to the gender.
Rinko and characters keep saying that she "became a man". What do they mean when they say this? They mean that her female gonads became male gonads.
If it ended there, then yeah, it feels like a bit of a stretch and is pretty open to interpretation still. But in the past few chapters we get:
- Her mom "realizing" that Rinko is a "boy" by looking at her skeleton?
- Not only is the premise ridiculous, but it's also a common transphobe talking point, which is being (in a way) validated through the manga's viewpoint by supplying the mother with "true" information. She can "see past" the dress, makeup, and composure, implying that the essence of being a "man" or "woman" is ingrained down to your bones.
- Her mother immediately links gender affirming care to invasive surgery. Claims that in order to "return" to being a woman, Rinko "has" to have surgery, and everyone just goes along with it.
- Again, a common transphobe tactic
- Also continues to link the gender to the genetalia - "in order to 'go back' to being a woman, you need to get rid of the 'man genetalia' and restore the 'woman genetalia'.
- Maybe, they're just talking about her status on the registry (was it even changed in the first place?) but that's never brought up, never been brought up before, and the mom goes on to imply that having a "fake" vagina isn't enough to "return to being a woman".
- The story beat in this last chapter makes it feel like this was the most critical we were going to get with these ideas. Rinko came to the conclusion that, "there is truth to both sides". Meaning there's no room to doubt the premise to begin with.
- Now we get a story that's framed as a: girl "becoming a man" (how? by growing a penis) and "turning back into a woman" (by surgically altering the penis into a vagina)
- Strengthening the link between gender and genitals
And yeah, any one of these things can be explained away easily enough. But when you look at them all lined up together, it starts to paint a trend that looks a lot like gender essentialism.
There's still time for the story to change perspectives and give alternate explanations, but my optimism was already starting to fade, and honestly, the more I think on the story and discuss it, the less optimism it seems I have for it to take a perspective that I would find convincing/compelling. We'll see.
Anyways, I probably could've made the same point a lot stronger with much fewer words, but I lack the eloquence, so I doubt I'll respond much further. I think I won't be able to make much meaningful contribution towards discussion past this point. Good discussing with you though. Thanks.