I think you can see from Annie’s first answer that it’s a bit of a difficult question to answer. If you’re “in the know” you don’t want to go into a huge rant and conclude with, to exaggerate, “and this is why your question is stupid and your ideas are wrong”. It’s not a stupid question, of course, and how are you to know all these things if you’ve never been exposed to them. Plus, not all trans or queer people agree with each other on these matters either, so … yeah. It’s complicated.
As for my two cents: The term “straight” makes very little sense in today’s time. Consider this: Antiquity did not have the concept at all; in Greece, you could fuck whoever you wanted, people didn’t care, so long as you did the fucking, not getting fucked (as a man, of course). Much the same in Rome, where giving oral was viewed as bad (as you dirtied your orator’s mouth, so to speak), but if you got head, it didn’t matter from whom. Plus, there’s a reason you have stories of people having traits of both sexes in ancient myths. Because such people have always been born and are still being born, but today, we still rarely hear of them.
Why’s that? Christianity came along and became the dominant religion. Sex wasn’t for pleasure any more, but to create offspring. So sexual acts between anything but men and women (this being, of course, vaginal sex in the bonds of matrimony) were viewed as wrong, because the Christian god is a builder who created things for a purpose (in this case men, women, and NOTHING in between). THEN you need to label those people who do things you don’t like, and voila, the homosexual is born. And ironically, you can’t have “straight people” without “homosexual people” as categories, like you can’t have light without dark, or cis without trans. My point is not that there weren’t men who were solely attracted to women before the term homosexual came into use, my point is that the category didn’t exist, you just had people. If you have heterosexuals and homosexuals, you also need clear lines between men and women. And nothing in between, because it would confuse your categories.
Add on top of that modern medicine, and suddenly, children born who were neither this nor that (intersex is a generally accepted term) were quietly assigned a “proper” sex. Via scalpel and operation. There’s a saying about these procedures: Less than one centimeter: A girl. More than three centimeters: A boy. Everything else: A problem. Parents weren’t even asked, and didn’t even know what happened to their children. Some doctors decided which was the easiest way to make a boy or a girl out of such “problems”, mutilating their perfectly healthy bodies in the process. Nature is much more complicated than “there are boys and girls”. But how does all this relate to trans people?
Well, they, too, don’t fit into those neat little boxes. Not everyone born with a penis feels good acting like society expects people with penises to act. Or people with vulvae, etc …In fact, for quite a number of people it’s sickening to act this way, or be addressed in a certain way, like Annie has pointed out. It’s not a fun life without the proper care.
From a point of view that posits that there are two sexes, period, this is problematic, just as intersex people are. Your body doesn’t make you a woman, or a man, but something happening in your head? There are people who stand between man and woman? What’s even going on?
I think “straight” makes a lot of sense as the opposite to “homosexual”, and maybe still if you throw in “bisexual”. But consider, just as a hypothetical example, a person being born intersex, let’s call them Alex, who does not get any kind of gender affirming surgery after birth. Alex doesn’t feel either as a boy or a girl, so they live as non-binary person. At age 16, they fall in love with a cis girl called Mary, and Mary falls in love with them. You can’t say this is a straight relationship, as no male person is involved, can you?
Anyway, they’re happy together and stay together. At age 21, Alex’ feelings towards their gender expression change and they decide to live as a man (i.e. act like a man acts because it feels natural and right to them at this point); however, as he is happy with his genitals, whatever they may look like, he doesn’t get an operation. Mary supports Alex, and they stay together. Is the relationship straight now? You could argue that yes, it is, because Alex now identifies as a man who is in love with a woman. You could say the people involved are still the same, so how could the nature of the relationship change?
Five years pass and Mary discovers she is trans. She transitions and now calls himself Martin. Alex stays together with Martin. Has Alex become gay now because of a change in another person? Is the relationship that might have been straight a homosexual one now? Or are we looking at something else all together?
I mean, you can always argue along these lines or similar ones. You can say “trans women are women”, and I would agree, but I still feel like our terminology for relationships is outdated in a way. Like wanting to know exactly what kind of Jazz King Crimson are playing. The existence of King Crimson doesn’t make Jazz classification wrong, nor is listening to Dixieland bad if King Crimson exists, but we need a different way to communicate about these things if we want to understand each other.