Showing messages to others

[InbetweenDreams]

Suppose if a person is in a relationship.

Is it acceptable that the person shows the private conversations between him and his love partner (e.g Emails, Whatsapp Messages, Facebook Messages etc.) to other people (like his friends, coworkers etc.) and let them read the messages without asking the love partner first?

Is it considered as a red flag in a relationship?

I think more context is needed. I think it is one thing to show a friend something cute that your partner sent you and so on but we're not talking about that are we? I would say in many cases it wouldn't be appropriate and could be a red flag.