Yes, that's right. They can't be friends. Unless both of them are involved with other people. Then they can. This is an amendment to the earlier rule.
— Harry Burns (Billy Crystal)
10 seconds sound clip from the When Harry Met Sally... movie soundboard.
You can hear this line at 00:21:32 in the Blu-ray version of the movie.
Quote context
[...]
- Would you like to have dinner? Just friends.
- I thought you didn't believe men and women could be friends.
- No, no, no, no, I never said that.
- If two people are in relationships, the pressure of possible involvement is lifted.
- Like it means something is missing from the relationship and you have to go outside to get it.
- I mean, come on, who the hell are we kidding? Let's face it.
[...]