gender identity and sexual orientation are different concepts, that have been married by European Christian dogma. harmonization in missionary work included harmonizing into a binary gender paradigm alongside a binary sex. many cultures across the Americas and Oceania had and have non-binary systems, before the swell of representation seen in the last decade or so.

although gender and sex is used interchangeably - even in the most progressive circles - gender is a reference to a set of cultural behaviors and roles, a form of expression, while sex is functional and 99.9999% chromosomal and binary in humans

you are familiar with this, for example, when someone says "be a man" in response to someone's lack of assertiveness, this has nothing to do with whether they have a penis and the binary male contributions to reproduction, it is referring to a behavior expression that is indeed arbitrary but shared

swapping genders therefore has nothing to do with what sex you are attracted to, when adopting that paradigm, especially when adding genders outside of the binary cultural behaviors

hence being "straight" doesn't change and is only a problem for someone else

> many cultures across the Americas and Oceania had and have non-binary systems,

As I understand it, this is because these cultures had deeply sexist ideas about how women and men should behave, so they created additional categories to shovel everyone who didn't conform into. In practice this tended to mean that gay men would be placed in some sort of "non-man" male category. So while sexuality and gender are different things, in practice they end up linked through this mechanism of othering.

You cant ignore sexuality in a fictional story about people changing their biological bodies.