You all aren't trying to explain why it doesn't make sense, you trying to describe how body-altering should work to your liking.
Usually, a sex changing works via *poof*-like magic or different kind of potions/drugs or hi-tech surgery. Realistically speaking, in most cases it's unlikely that only the brain will remain perfectly intact while the rest of the body changes. It's much harder to get it to work that way, maybe with except of surgery case. Much easier to change whole body, resulting in a completely different person, as you mentioned, and different sexual preferences.
So my point is: if you want to and try to find some sense here, you will. If not, you can proceed with whining about why something works wrong.
The question is a bit metaphysical but the way I see it, there's two major points:
- Do you regard the brain as being you, or is it just another organ?
- Do you think sexual attraction is purely a biological response? Something learned? Somewhere in between or something else entirely?
If the brain literally is you, then having body-swaps change your brain means you effectively died and were totally replaced. The person that's there is just someone with your memories; Unless you think you are nothing more than a set of memories, but that's another discussion more relevant to cloning or whatever.
If the brain is
not literally you (something I subscribe to, considering the myraid of problems I get that come from my brain and mind having their own ideas as to how things should run), then the brain can be changed and still consider you the same person, and the second question can come into play... but the second question leads to some nasty places that have relevance in the real world and aren't all that fun to talk about. If you believe it's purely biological, then it's something that can be medically "fixed" or screened for (and homophobic people will do this). If you believe it's purely something learned, then it's implying that people are taught or conditioned into their sexuality (and homophobes and transphobes (but I repeat myself) already seem to be assuming this and trying to hunt it out). Assuming it's a combination of the two just subscribes to both viewpoints, but "something else" is also a cop-out.
That said, part of what makes gender-swap stuff interesting, in my opinion, is the conflict between the character's old mind and what the body wants to do. Generally, I think it should take the "The mind is an organ, sexual attraction is a mix" approach, where their conditioning (/what they're used to) is coming into conflict with what the body wants/reacts to. It shouldn't just be "OMG I LOVE DICKS NOW" (not in the least because that approach is boring and a waste to begin with, the people you're trying to appease will still complain it's gay anyway, just write a cis girl coming out of her shell to slut it up instead) but the person shouldn't be 100% their old self anymore, either.
