No, it's not that. It's just you don't know me.
4 seconds sound clip from the Shockproof (1949) movie soundboard.
You can hear this line at 00:40:07.580 in the Blu-ray version of the movie.
Quote context
[...]
- I guess I like mostly what you might call simple things.
- I can't offer you the expensive gowns, the expensive times that Harry Wesson can.
- But I do love you, Jenny. Will you marry me, Jenny?
- Oh, no. You don't know what you're saying. It's no good. It couldn't work.
- No, it's not that. It's just you don't know me.
- You know something? You're the first man who ever asked me to marry him.
- I know we can't get married right away anyhow, until... But you could say yes now, couldn't you?
- Okay, Jenny, you think about it, but you know how I feel.
[...]