Absolutely necessary.
4 seconds sound clip from the White Christmas (1954) movie soundboard.
You can hear this line at 01:23:50 in the Blu-ray version of the movie.
Quote context
[...]
- Look it would only be temporary, huh?
- Well, you don't think I'm the kind of girl that goes around throwing myself at a man, do you?
- Of course not. Nobody thinks that.
- And we wouldn't have to announce our engagement till it was absolutely necessary.
- But don't you think we ought to kiss or something?
- Um... Not until it's absolutely necessary.
- Bob, would you like to dance?
[...]