Responses
So, part of it is still the respect aspect. The Korean culture in a way is more rigid than Japanese culture when it comes to respect (they have technically 9 different modes based on who you're talking to, but its mostly just three used in everyday language). But in the case of this story, he still calls him Sunbae because Jiwon kinda loses it and fucks him silly when he calls him Hyung. For the preservation of his anus, I can't imagine what would happen if he called him by his first name.
I hate that he still calls him sunbae and not by his first name jiwon ☹ Arent they past the whole sign of respect for your senpai thing since there dating?
(And ik senpai is japanese, im just giving an example)