Oh i can see the people calling me racist for what i'm going write, that said I'm someone of mixed race (very) since in my country there was not interracial marriage ban.
My theory works of two components of beauty, you see beauty is a multifaceted feature, some people are beautifull because they are cute, some people are atractive because they have a lot of sensuality withim themselves, some people are atractive because they are manly/femenine and some people are atractive because they got really simetric features.
those two components being Cutenes and roughtness that can be directly asociated to masculinity. Most people have a little bit of everything but there are some extremist that given an example, have 0 cuteness, but they are rought/manly as hell and that makes them atractive, works the other way around too.
When we talk about top and bottoming, heteronormative features/roles within our biology arise and manifest and we often see that Generaly speaking, Homosexual top beauty turns around roughness and a lot of masculinity, and homosexual bottom beauty turns around a lot of cuteness and a little bit of masculinity/feminity mixed.
Given example, you see this men and what does he ispire you? he is clearly not the most handsome guy, on the world, 0 cuteness, but he overflows with roughness and masculinity, and you would most likely end on your kness saying "wharever you want daddy" if you see him
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now lets see this one
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This one, fairly masculine, and absurdly cute men... what does he inspire you to do?... most likely you wanna split him in a half and cuddle him after while you whisper him "everything is going to be ok babe"
All that said, black people loose to white people on the cuteness department 8 times out 10.
Also black people win on the roughtness department wich is an key feature for the homosexual top beauty and that can lead the the current situation on porn.
Btw, that would end just on discrimination for black bottoms, and that's already evident.