Are you "just friends" if you say "I miss you" and writing poems to each other?

If the other person is saying that, I would take them at their word.

I say I miss you to my friends all the time
And I’d write them poetry if I could
I do my own equivalent which is long rambling messages about why I care about them 😂

No, I don't believe that is "[i]just[/i] friends". Someone is lying to themselves. I've never written a romantic poem to a friend [b]ever[/b].

You want to f*ck each other but you just won't admit it.

yes

That sounds like friendship to me.