Well, it's uncomfortable because how it depicted the SA and tried to pass it lightly. BUT, here's this:
1. The artstyle is not sober enough, it's still quite colorful and it makes it seem less serious.
2. The discomfort is real, the MC isn't happy and the small gestures signal it. The artist actually make good use of symbolic elements to show how painful it was what his brothers did to him.
3. The MC is promiscuous and a past life tyrant. His psychology about life may be twisted or he can't give enough importance (if it's not related to the ML) since he has seen his share of bloodshed. He doesn't hold his body to the same value as others since he uses it as a means to an end.
4. Although these SA scenes are uncomfortable and seem unnecessary, it's how he does it. Sometimes, we have to accept that's the character's methods, not some witty white lotus act or green tea machinations, or direct use of the ML abilities. Besides, if the depiction was even more serious, we have to acknowledge that the world setting is quite liberal regarding sex and that such strategy can exist and islts precisely used by the MC. If this was a novel, I'm sure it would have sounded more angsty, but the manhua has more of these comedic relief and wants to focus how the MC solely wants the ML's love more than his care for the world.
The probability of Jei also being aware of Seokjin's feeling but acting dumb (and even getting goddarn engaged) as petty act of him never taking the initiative is also high ah.
Even if he doesn't explicitly think so, sometimes humans are petty without them knowing for things they take for granted. He takes for granted Seokjin will always be by his side, even if only a friend, and Seokjin as well, is doing his best to keep the friendship at the cost of heartbreak.
Anyway, Han Jei deserves no sympathy for leading on a lady.