There shouldnt be any middle ground on the statement "trans rights are human rights". If you think there's room for compromise on the oppression of marginalized groups then you're either a lost cause or willfully ignorant of the world around you. People that are in a safe space, specifically dedicated to them and people like them, have no obligation to educate or entertain disingenuous "discourse" about whether or not they should be allowed to exist.
Gotta say, pre-emptive defederation seems like a bit of an over reaction. There's plenty of leftists in other instances, not just hexbear and lemmygrad. Hell, I'm an anarchist that regularly comments on political/social threads in this instance, it doesn't seem to be an issue. Lemmy in general is pretty obviously far left leaning. I don't see the harm in federating with hexbear, unless they show themselves to be actively harassing or trolling. But you'd have to federate first to find out if they will