I suggest this because, firstly, it's necessary for us to both contribute a curve25519 key (and agree to forget the private half) so I need to send you something first anyway (so make that an "Invite")

And secondly, there is a balance of trust between sharers of intimate secrets. I don't want anyone to get unsolicited off-the-record threats. Inviting you to send me something off-the-record is an expression of trust, and you've gotta trust me that I'm not gonna repeat it.

(because even if there isn't any cryptographic evidence - if it can be corroborate, then it's still leakable)