When you message someone on social media, do you not assume their partner is reading their messages?

So, I'm having an argument with my boyfriend right now.

I believe I should be able read his messages but he disagrees, says that the people he messages send those messages to him, not to me, and I am not part of the conversation, and if I want to read them I have to get the other persons permission to read them, but I disagree.

You should always assume someones partner is reading the messages too, in any decent relationship.

When you message someone on social media, do you not assume their partner is reading their messages?
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