if you see some of the kissing scenes between NK and SG, and focus on SGs expression, it actually looks like he feels a lot and trying to convey that through his body.
(spoilers)
He was saying it as a defence as he knows full well that is not the truth but doesn't want to expose his feelings for Nakyum as he believes they are unrequited, just like he disguised his care for Nakyum's health as him being too skinny for sex...I hope next chapter they are both just honest with each other（/TДT)/
no some be embarrassed i thought the exact same thing
idk what's the correct term but i think he was saying it ironically? mockingly? basically he didn't actually mean it but he's not brave enough to deny what nakyum said (why he keeps him around) because then he'd be opening up in the process. so he's just playing to what nakyum said but he's not happy about it either. i suspect that's he's taking it out on nakyum while they're having sex because he's frustrated
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maybe it's just me or was seungho trying to deny something more than physical by saying it again and again that he only kept nakyum by his side for *ahem* recreational purposes? it felt more like he was trying to convince himself. but who knows, our author can pull up some plottwist out of the blue any second. i'm lowkey embarrased by this comment lol