When Lumine enters the Wangsheng Funeral Parlor to get some infos out of Zhongli, she just happens to be witness to Zhongli pulling Childe into a passionated kiss.
Zhongli & Childe: were busy kissing each other
Lumine (internal): What are you doing? Are you putting you tongue in his mouth? Why are you putting your tongue in his mouth? Why are you doing such things in public? Eeeww!!!!
Lumine (external): looks very shocked, confused and traumatized.
And then Lumine goes to Xiangling and tells her about what happened and during their conversation Xiangling learns that Lumine thinks kissing is disgusting and it makes her highly uncomfortable to see other people kissing each other.
Word gets out and so Jean/Diluc, Beidou/Ningguang and Zhongli/Childe meet up to discuss this matter. Why those three couples? They all had been seen by Lumine during their “public” kissing session. Also Jean/Diluc feel responsible for the young travler from a different world. Zhongli was no clue what’s going on, but hasd been dragged into it by Childe, who sees Lumine as a friend and secretly is worried about her.
They are all stupid, but act like their are smart. It ends in chaos and maybe a even more traumatized Lumine.