prim and proper
Having very traditional, morally conservative beliefs and behavior. I've never dated someone who is so prim and proper before. I'm not sure how he's going react when he meets my family! She's so prim and proper that I bet she's never even jaywalked before.
prim and ˈproper
(of a person) very correctly behaved and easily shocked by anything that is rude: Don’t invite her to the party. She’s so prim and proper.