The fact that it wouldn't have been considered legally rape in the middle ages (tho' it would, but Cersei would have been burned at the stake for tempting her brother into wickedness and sin) is a cop-out in a show with dragons, shadow-demons and whitewalkers. It is a fantasy show, not a history show, and as I pointed out, if it wanted to be so historically accurate, Gilly would have been able to seek refuge in a cloister rather than a brothel, Cersei would have been able to use culture to cement her power, and women would have been brewsters and midwives - women in the medieval world had choices beyond upper class women were brood mares and lower class women ended up as whores. Heck, even the septas, of whom we have seen one or two, are historically inaccurate - nuns wouldn't live with families, but the families would send their daughters to monasteries, and they certainly wouldn't have accepted the way Arya disrespected them.
I understand that you love this series, and you are completely entitled to do so, regardless of its problems. If we were to refuse seeing things that are problematic, there wouldn't be anything to see. But we are also allowed to point out what is problematic in a tv series, without seeming to condemn it completely. The tv producers use of rape in this series is problematic. They use it as they would fridging a female character - to illustrate the character of the men. The women are never shown dealing with their rape, they are there only to be raped. I am sure they had banked on Cersei's unpopularity to protect them from a backlash - and indeed, most of the complaints are about the treatment of Jaime, rather than Cersei. Just because Cersei has found a way to survive in King's Landing, and survive being forcibly married (marital rape beign very typical in GoT) by carving out a little haven of consent with her brother, doesn't mean that she cannot withdraw that consent at any time. Which she does.
The scene depicted a rape. Cersei was raped. Jaime raped her. Regardless of what their relationship was prior to this, regardless of Jaime's inner motivation, which most certainly did *not* come out on screen, regardless of Martin's "historical accuracy", the fact remains, Cersei said no, and Jaime ignored it. That is rape.