Read Miss, Something's Wrong With You Manga:
This is the typical protagonist setting so my cheats should be coming, right! Unexpectedly, that night, my fiancée suddenly took back her words: I won’t annul this marriage! I want us to live and sleep together! I’ll also take care of all our living expenses! Okay, let’s do it, I have nothing to lose anyway. After one night… Eh, there’s something a bit w more.. rong with my fiancée?