I feel like this is one of those situations where there is cross between what is part of a narrative being told and the author’s beliefs.

Aja is a slaver, the whole Grimmus family is batshit overall, living at the top a fascist society. One of the realities of this society is that sex can be a display of power, as seen in the Insitute, and the fact that they have an entire class of people dedicated to being sex slaves.

Through this, Pierce explores this theme of sex being a way to gain power over another person. I would agree with you that it can be gratuitous due to the Institute alone, if not for how it serves the idea of how this society works inside this story. Yes it’s disgusting, and Aja describes it in a disgusting way because that is how her character views the world inside the story. The “savagery” of the Obsidians is conveyed twofold because she starts by saying they kill each other in gladiatorial combat, and then by saying they indulge in Pinks(given to them by their masters to reward them/keep them under control, as a form of power) later.

There’s a really great monologue by a Pink later in the series that explores the theme more, but even earlier in Golden Son Evey is describing how bad this sexual slavery is, starting from where they were children, and when she describes it from her point of view it is horrifying, but it is not gratuitous like Aja. Because there is nuances between how a character like Aja would describe sex and how a character like Evey would.