If we operate with the assumption that Nanjo is still only entertaining Mokyeong bc of Sunhwa...
He's confused as to why Mokyeong is showing him concern. Their relationship is contractual, not based on emotion. So when Mokyeong expresses his concerns about Nanjo continuing with the arrangement, asking if Sunhwa really means so much to him that he'd continue to be butt fucked by a man he so clearly does not like (and who is also steadily growing obsessed), Nanjo's response is kind of a sarcastic "I didn't know my feelings held any weight within this agreement." Or like a "Since when did you ever care about how I feel?". Basically saying to Mokyeong, well you came to fuck, so fuck me and go.
He's making sure Mokyeong knows that there is no warmth or affection involved in this, that he's simply fulfilling the terms of their agreement. Whether that's really true or not... ¯_(ツ)_/¯
i think a more localized translation of that sentence in english (rather than a literal translation) would be that mokyeon says something along the lines of “is seonhwa that important to you? for you to endure this much?” and nanjo replies with “what’s it to you?”
(comment above did a good job conveying the tone but this is just my two cents from someone who read the raws)
what did nanjo mean when he said “does my well being have any meaning?”