It's not that someone can forward your mail unencrypted. It's that in the normal operation of the system, someone taking the natural next step in a conversation (replying) can --- and, in the experience of everyone I've talked to who has used PGP in anger for any extended period of time, inevitably does --- destroy the security of the entire conversation by accidentally replying in plaintext.

That can't happen in any modern encrypted messenger. It does happen routinely with encrypted email.