HOW DO PEOPLE LIKE THIS???

Tae-oh made it very clear from the onset that he's a freak. Dong-jae was even creeped out by him at first and turned him down. Over the course of the story Tae-oh falls in love with him and his freak remains, but the lust comes from a place of love rather than just for his body like was the case at first. The point being that Dong-jae understands his intentions and him being a pervert is sort of just part of his personality, a part that Dong-jae has accepted more and more until eventually he's refusing to let him disappear and choosing to date both of them.

I also want to point out that Dong-jae is written in a very fetishy way as well. Very innocent, but with a smoking body. Prudish, but weak to pleasure. The story even has him talk about how "disgusting and dirty" it is for Tae-oh to touch his asshole in that final sex scene where they all finally go all the way. He's a naive virgin learning gay sex. It's very in-character for Dong-jae to get drunk and innocently mess around with Tae-oh's dick and for Tae-oh to not bother trying to resist in the face of that.