We see that Sebin was fully aware from the beginning that Shinwoo was longing for someone ...

We see that Sebin was fully aware from the beginning that Shinwoo was longing for someone else. He decided to continue despite that.
He probably hoped that over time the feelings between the two of them would be enough to overcome that. And there's no doubt that he did end up being played and taken advantage of, and used. But he has also been aware of most of these things to a certain degree the entire time. He didn't deserve them- but he did choose to stay knowing that they existed, just so long as he felt that there was the slightest chance.