From what I remember she wasn't bothered by it at all until that point unless sierra changed it? In fact, as I recall, when she was the literal cumdump for the soldiers, she even encouraged simon to abuse her, the reason she was upset at him was because he didn't do it.
She wanted specifically Simon to
use her, I don't think she ever displayed any enjoyment towards the idea of being
abused. She is already infatuated with Simon from the game's start, we don't really get much context as to why and in a first playthrough one might assume it is because she is just
that slutty, but after learning more about the characters it is more likely that Simon was the only one to express any concern for her, which made her start to fall in love with him. Bit of an accidental stockholm syndrome but Simon genuinely has a good heart so it worked out in the end.
Then immediately after leaving the town, you're ambushed by slavers, and when you "rescue" Qum from him, he complains she just can't get enough sex with him.
Qum asks him "Aww, that's all you have for Cumdump?", slaver guy says "What... what are you? My balls are so empty...", Qum then says "Simon! These dumb guys are being mean to Cumdump!", then you both proceed to beat them to death, and Qum says "Dumb guys are boring. Cumdump wants to be just Simon's forever and ever!"
I don't think the implication is "Qum couldn't get enough sex with random slaver dudes" as much as "Qum couldn't get good sex with random slaver dudes". I don't know if Qum actually meant to taunt the slavers or if it was just an innocent expression of dissapointment, but I'd say that ended up strengthening Qum's attachment to Simon if anything. Also I think for Qum there is a difference between being dissapointingly fucked by some random chumps for a few minutes before being rescued by Simon and being gangbanged by rough crowds for several days because Simon pressured her into doing it.
Then you're given an option to whore her out. And for whatever reason it's the bad option, and she's unwilling. I don't even know why simon is given the option to do this, it's completely against his persona.
Early game Simon is not as infallible as he becomes later on imo. He was still ignorant about some aspects of succubi culture and physiology, there was much uncertainty about how far their plans could go so every bit could count, and on a meta level the dev was okay with allowing some more freedom of choice in the early game and exploring Simon's ruthless side.
Then at the gathering she seems perfectly willing to fuck orcs and ogres with yarra if you don't whore her out so idk why the qum whored was such a traumatic experience for her.
Those scenes have very different moods. I think the main thing to understand about Qum's trauma is that it is not tied to sex, she's a succubus so that part is easy for her, her trauma is tied to abandonment issues and harsh treatment.
For the first thing, Qum's issue with being whored is that it essentially means that Simon was willing to part with her and leave her isolated from the party for several days for a small lump sum of money, if Simon was willing to do that, would it be inconceivable to think that Simon would be willing to abandon Qum and accept to sell her permanently if some eccentric rich noble showed up offering 1 million ProN? Probably, but it is now a creeping fear in the back of her head.
For the second thing, Qum just doesn't like abusive mean-spirited stuff, and ironically enough the scenes with the orcs and ogre are much nicer than the brothel gangbang, the orcs treat Qum gently and the Ogre sits quietly and just enjoys Qum and Yarra making out on his cock. Partially that was the case because Yarra was there to control the situation and keep things orderly, but maybe the point was also that succubi cultures just have less hangups about sex than human cultures, maybe that makes them behave better when they just want good sex instead of getting super obnoxious about it.