What pleasure ? Did you see how he f*cked him ? Do you even know how the ass works ? It’s definitely not like how many webtoons portray it —people having orgasms the moment they’re penetrated or as soon as you reach their prostate, lol. It's not instantly comfortable or enjoyable for everyone. The body area involved has muscles that naturally tighten, and pleasure depends on so many factors like relaxation, patience, and adequate preparation. So considering how Uno is not a bottom to begin with and that none of those conditions were present in the situation, it's more than obvious that he was experiencing discomfort and pain rather than pleasure. Heck, he said so himself on more than one occasion !
Uno was definitely hurting badly; there’s no way he felt pleasure, let alone "gave in" to it.
The funny thing about ‘reading between the lines’ is that you still have to read the actual lines first. The text, the character’s actions, and the context all point to the same thing — you’re just choosing to overlook it.
The artist spelled it out and yet you’re still arguing against the character’s own words and actions. At that point it’s not reading between the lines, it’s rewriting the story lol
Also, using badumX5 as your main argument is sending me xDD A heartbeat sound effect can represent fear, stress, shock, panic, or any intense physical reaction. But yeah, sure, in this situation it obviously means he’s secretly falling in love. Doki doki ~ (⌒▽⌒)

He doesn't want to give in to the pleasure, it'll rip out his remaining dignity as a top! (≧▽≦)
But I understand bcuz it hurts as hell to surrender to someone who raped ya~ but hey, he could have fought back but couldn't because of that pretty face~ ┗( T﹏T )┛