What if Luhan kissed Xiumin
Like literal lip to lip contact? I'd be watching for their expressions, and depending on their expressions is how I'll react.
For example, like with Minho and Jonghyun on the bleachers that one time where Jonghyun was playing around and leaned forward feigning that he wanted a kiss and Minho lightly kissed him just to shock him. I laughed at that because of Jonghyun's shocked expression, like he was thinking, "HOLY SHIT THAT WAS UNEXPECTED." lmfao
If they were really nervous about it or something like when Daniel and Youngwon were playing that game in the car and Daniel ended up kissing Youngwon because that had to finish the game and Youngwon was freaking out I probably would just go, "AAAAAAWE hseriugkdjf"
tl;dr - I'd laugh if Xiumin was shocked, go aaaawe if he got flustered.