Does this "how could I decide to do sth so cruel" mean yangyang wants to reject him?

I think he meant that it would be cruel to date him, maybe because you go through a lot when you are homosexual, or something like that

Then, is he saying he's already in love? If it's like that he should just let it be!! (▰˘◡˘▰)

I think he’s talking about denying li huan’s feelings

I agree with Lolatako above. I also think YuYang doesn't want LiHuan to go through the "burden" and troubles of being a homosexual in this story, since YuYang had to and still has to experience all these troubles himself. He probably is trying to be kind to not let LiHuan deal with this, but he's also a bit selfish in the way that he just doesn't want to take LiHuan seriously because of personal reasons (maybe like it's too good to be true that someone actually likes him, insecurity issues, or whatever else). Let's just wait for LiHuan to win him more. Have faith in our big guy!