I recently realized that the straightness category is very analogous to whiteness.
If a cis person were to date an agender enby, they might be heterosexual by virtue of being sexually attracted to someone of a different phenotypical gender/sex, but would probably not be considered straight. Despite the enby being hetero to the cis person, people might call them homos, even if the enby was AMAB and the cis person was a woman.
This is why people see attraction to trans people as not straight. If the chick has a dick or the dude doesn't, it just doesn't feel straight to cis people. Even if the trans person had The SurgeryTM, the fact that they're not "normal" makes attraction to them not straight to these freaks.
Just like conceptions of whiteness, male/female, and pornography, straightness is really based on vibes rather than strict criteria. This allows the category to be used as a tool of exclusion and oppression, given legitimacy by its supposed ties to objective measures.
Straight =/= heterosexual. Liking femboys is gay, liking tomboys is gay, liking trans people is gay. An AMAB transfem & an AFAB transmasc in a trans for trans relationship aren't straight, even though they aren't homosexual from any coherent perspective. Straightness isn't coherent beyond separating normal from queer. All heterosexual people can find a trans person hot if we pass, and many will find us hotter if they know we're trans.
I'm bi with obvious bias towards femme presenting people, and some scummier gay men (those that forgot 2000's homophobia and now are transphobes on the same tropes) did harass me for being a "spicy heterosexual into getting pegged".
I think part of the issue with these terms losing meaning is because we stopped using them as intended. Straight Gay and Bi were just the entry points in delivering vital info about what sexual organs you had and which ones you were engaging in sex acts with. Under old school definitions that would list me as Straight because I have a penis, and am only interested in engaging in sex with a vagina. Now, because it does not matter to me whether the person who has a vag is a man or woman, under the newer definitions I am bi. So the system made sense for the original system we set it up in, but its breaking down in our more modern way of thinking
edit: Side note, I especially like how this effectively makes me Shroedingers Gay, as I am either Straight or Bi depending on whos looking
They made sense in a world where trans and gender nonconforming people don't exist. In this mythical past you imagine, you having sex with a masc presenting woman wouldn't be straight either, as you'd be having sex with a queer freak. It's never been about genitals.
Anal sex, hetero or homosexual, was sodomy, and while they usually went after homosexuals, they would be willing to go after "straight" people if it interfered with the woman pumping out babies. If you're visibly not fitting into the mold, people will deny you're straight. Many people don't mind, but the conservatives would throw people out of thefi group if you "feel" off to them.
Trans people have always existed and GNC people have always existed. The old understanding was never accurate intentionally. It excluded people by design, and is only losing meaning because we're actually questioning it. When it was not questioned, it wasn't somehow more accurate. We're not creating new definitions, we're pinning down the unspoken parts of the original definition.
I'm not trying to minimize anyones life experience with my previous statement. We try to improve society as time moves forward, and if we are successful at that, that means that in the past things weren't as good/inclusive/knowledgeable as they are now. Public knowledge on sex/gender/biology has advanced a lot since we started using the terms straight gay and bi, I was merely acknowledging the past existed as it was, and that they worked for the more limited public knowledge that we had then, we know more now and they've become less relevant/useful because of that. The one thing you are wrong about though is yes, it WAS primarily about genitals, if you had a dick and liked dick, you were gay, same with if you had a vag and liked vag. If you liked both, it was bi, and if you liked the opposite it was straight. Society as a whole didnt fully understand the difference between sex and gender then though, so what genitals you were interested in and what gender you were interested in were used interchangeably, but that didnt stop the primary purpose as it was generally understood back then from being specifically which sexual organs you wanted to do the nasty with.
Tl:dr: as we advance our understanding of our world and old models become inefficient/outdated, it doesnt mean they werent compatable with our modern understanding of the world out of malice, it just means we have advanced to the point they no longer work
That's it, absolutely. It's bigotry in language form. The only people who care about what qualifies as "straight" are those who shun those different from themselves.