I don't think it was a definite 'r' scene. More like- forced at first and consented later. I think the writers just wanted to show how screwed up Ta Hwan and Nyang really are. I am actually relieved they didn't have Nyang acting like a victim. That would be even more terrible. At least we can defend the scene in some way after seeing her being normal after everything happened (if it really did).
Still, the writers are now just using cheap methods to create drama. I wish it gets over soon. When things go too crazy in a drama, I always wish for the characters to end up dead lol. Seems more fitting to me.