Be Your Knight Episode 4

Cheng Che learns that Ye Ran came back to his family for a personal revenge and she is afraid that he will hurt Xiang Yiru (Xiang matriarch), but then he relieves her worry by explaining that he is one of the Xiangs despite not living in their house, so he won't hurt her. They have a wedding ceremony. Cheng Che doesn't want to kiss Ye Ran on the lips during ceremony but to make it more believable and less suspicious Ye Ran grabs her face and kisses her on the lips. After the ceremony when they are alone she slaps his face for that kiss. (Source: KamilaRN at MyDramaList)