[Grey Wind]

Even though I think rape isn't as black and white as everyone thinks, the point of it was Jaimie expressing his dominance over Cersei. He used to be the best swordsman in the 7 kingdoms, that's what largely attracted Cersei to him in the first place. After he got captured and lost his hand, she proceeded to call him out on how he was less of a man because of it. She even calls him less of a man than her. He proved she was wrong by fucking her. It's not 'right', but in such a horrible society, people solve issues by doing horrible things.

It ruins Jamie's character arc if you're the uptight type who moans about rape culture, but as everyone else said, this is a show where people do much more horrible things. But peoples biases cause non consensual sex to be more of an outrage than murder and war, unfortunately.