The question assumes we know what someone believes before we've spoken to them. That's the actual problem here, people are being excluded based on assumed beliefs rather than demonstrated behaviour.

Opinions evolve through exposure to different viewpoints, not through isolation from them. The homophobes and racists of the 80s who changed their minds didn't do so because they were shut out of communities - they changed because they were forced to actually interact with the people they'd made assumptions about. That contact broke down the assumptions.

When you exclude someone pre-emptively because you've decided what they must believe, you've eliminated the possibility of that evolution happening. You've also replicated the exact mechanism that made 80s bigotry so pernicious: denying participation based on identity or assumed characteristics rather than actual conduct.

Everyone thinks they're right. The racists thought they were right. The homophobes thought they were right. You think you're right. I think I'm right. That's why behaviour-based boundaries matter more than belief-based ones. Judge people on what they actually do in the space, not what you assume they think.

If your moral framework requires everyone to already agree with you before they're allowed to participate, you're not building a community - you're enforcing an orthodoxy. And orthodoxies don't evolve, they just calcify.