I'm going to try to take a stab at this without resorting to whether the scene canonically was or wasn't rape- all I can say for certain is that it is eventually consensual. In either case, it's a disturbing scene on a number of levels, especially considering the emotional context.
What disturbs me far more about the show's portrayal of the scene is the light it casts on Cersei and Jaime's relationship as a larger whole. In the end, they are both two undeniably broken and ed up individuals, but their relationship hinges on personal choice. Loving Jaime is a choice Cersei made, and that choice is especially important when you consider that throughout the years of her marriage, throughout years of suffering rape and abuse at the hands of Robert over and over again, Jaime was the lover she chose. She was not auctioned off to him, she was not coerced into a relationship with him- their relationship, as messed up and unhealthy as it is, is a representation of her own agency as a character. She is not Jaime's victim. She is his equal- and often, his better.
In the other corner, Jaime is someone who witnessed first hand the effects of abuse in a relationship that forms a distinct parallel with his own- the marriage of Rhaella and Aerys. And the abuse he heard and saw in the Red Keep has haunted him for decades. The nature of Jaime and Cersei's relationship is not the same as the nature of Aerys and Rhaella's. That is established.
So when the directors and writers make a distinct choice to frame this scene in the way that they did, with no verbal consent, with Cersei clearly resisting, and with Jaime as a malicious, vindictive aggressor and abuser, it begs the question of why. What statement were they trying to make? How do they see the relationship between the twins? And why did they see it as necessary to change it?
Rape or not, consent or not, this was a deliberate change from the book. And I can't wrap my head around any reason why that change is a good thing in the larger context of Jaime and Cersei's relationship.