Grian sits on Joel's heart bench for some good old gay roleplay and Joel goes "no, no, no, I'm married". We all know what he'd have done if Jimmy were there instead, so—this isn't about being married. Grian specifically isn't safe to flirt with for Joel because Grian would take the reigns in that relationship and never let go. He'd go full bully and make Joel as uncomfortable as possible and watch him squirm as he keeps poking at him. Because Joel is a total tsundere there's nothing he could do to escape that narrative. It's the last thing Joel wants. It's why he's not letting go of the edgy persona: to stay in control.
Is it though? That doesn't seem to give him enough credit. He plays into the angsty teen role as him being pathetic so he's self-aware to a point. He doesn't actually do everything in his power to be seen as tough and badass and nothing else. I'm pretty sure him being soft and caring is part of what his and Lizzie's relationship is built on. Ah.
It's too close to what that relationship looks like. He can flirt with Jimmy (or Etho or Bdubs or whoever) because he's in charge there, mostly. He can't flirt with Grian because that's what it means to be married. He literally said it. Post cancelled, I've been running in circles.