I think after he got sick from dicking
from an outside point of view his attitude changed towards them
at first he was still seeing his fuck buddies and NK was only there to draw
now he cut off hooking up with them and only do it with NK
I think Seungho's attitude changed towards Nakyum when Nakyum was drunk and slept with Seungho because he thought he was Inhun. Even though Seungho knew that he was mistaken for Inhun he still felt some type of way when Nakyum said I love you and was all lovey dovey. I even think it was might have been before that because he was a lot gentler to Nakyum than anyone else.
It didn't
Please remind me again please, when did Seungho actually change his attitude towards Nakyum?