
Unpopular/Jaded Opinion:
I think one of the reasons for this is self inserting. *Some* people tend to pick a character (usually the uke) as their audience surrogate. Because of this they don't mind if he is showered with attention, romantic options, etc. But if it's the character that isn't the audience surrogate (seme, other ukes) getting the attention they won't like it. A good example of this I think is 19 Days. Once He Tian (black hair) got paired off with new guy Mo Guanshan (redhead) a lot of people got mad because it wasn't their reader surrogate even though he already has his own partner. I expect we'll see a similar situation here. You see this a lot with shoujo & harem readers too. Maybe this won't make sense to everyone but I really believe a lot of anime/manga fans do this.

I have this sme theory, and I am almost certain it is correct.
I don't really understand how most of the people are okay with that new guy (no hate) but if it was Xiao Hang who found a cute Uke everyone would be hating on him for sure like how can you leave Ye Zi alone and all that bullshit.
If it actually turns out that Ye Zi went out with that guy (the reason Xiao Hang looked mad when he saw them together) then he definitely deserves that treatment from Xiao Hang bc unlike him Xiao Hang make it so obvious that he likes him (calling him the prettiest infront of everyone and he is going to protect him and all that).
This is my own opinion you don't have to agree with it (no hate!)