Why is it normal to create hetero couples without development in fiction, but it's strange if they are LGBT? ?

Why is it ok to create heterosexual couples in a fictional story without having to explain why they are heterosexual, but is it necessary to explain why a couple is LGBT?

I'm not talking about character development or story development, I'm talking about the simple fact that they are who they are. I always see people asking why a character is homosexual. Which I find strange, because the answer is obvious, he is homosexual, because he was born that way in the same way that heterosexuals were born heterosexuals. Why do they require an explanation for the character being homosexual (I'm not talking about personality development) but don't when the character is heterosexual? If they see a heterosexual couple, they don't question anything, but if they see a LGBT couple, they demand that there be a past, a trauma, etc., that justifies the character being homosexual. Why does this happen?

It seems the same with black characters. It's ok to create a story where the protagonist dies and is reincarnated in another universe and, coincidentally, everyone there is Japanese or white, but if they created a universe where everyone is black, the same people would say it's unrealistic for all human characters to be black in another universe.

If they want people to create their own characters, why do they attack when they do it the way they want?

Why is it normal to create hetero couples without development in fiction, but it's strange if they are LGBT? ?
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