Some people do have unchangeable sex preferences, though. Should that not be reflected in the game? There are plenty of other characters who are versatile/can be talked in to other positions, why is it important for Katari specifically to be?
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Regarding Katari, he describes the Uruk species as being a very serene and patient race. He is placid, peaceful, kind, tolerant and loving, he also admits to being a needy hugger.
With that kind and affectionate personality, Katari would want to pleasure his partners as much as possible and he would most likely be sexually versatile. He seems to be open to new things and not be intolerant or unwilling to experiment.
When he discovered proof that Rylan was responsible for disabling the spaceships defences, weapons, engine core and providing vital information for the initial attack he became hateful towards Rylan with good reason.
However I believe that Rylan did not know there was going to be such a big loss of life, he was expecting a more precise attack aimed at getting the research documents and the scientist and then escaping quickly.
Bradley's cold blooded ruthless nature and brutality is why Rylan turned on Bradley as he knew that he was an out of control psychopath.
Rylan seems to have been convinced by the mc/captain of the need to resolve the strong feelings of hatred towards him. So Rylan and the mc/ship's captain staged the sexual assault in an attempt to appease Katari.
It is understandable that Katari wanted justice and revenge for what Rylan did.
When Katari and Bradley forcibly raped and abused Rylan in a cruel and brutal manner, that was totally out of character for an Uruk. I believe an Uruk would have wanted justice served via a fair and proper trial and if convicted a very lengthy prison term.
This was made worse when Katari admitted before the brutal rape scene that he had found PROOF of what Rylan did BEFORE he accused Rylan of sabotage in the lunch room.
So why didn't Katari show his evidence of Rylan's guilt back then?.
Having more characters being sexually versatile would make the game more interesting, for example Galiano is initially portrayed as being extremely dominant and a top only but I understand that later in the game he can be persuaded to be passive/bottom or even turned into a bitch and rented out like a common prostitute.
In reality some people do indeed only top others sexually, but a few can be persuaded to change and become more versatile, even if that only happens very rarely.