I dont think they should preclude or discourage participation from anyone either. I'd like to hope that a lot of the time, they don't. Not unless it specifically says "no boys allowed".

But its my understanding that if you want to be an ally to some sort of group, that its better to approach things with patience and humility, because they know better than you/I do about what its like to be them. Kinda like how its best to lurk a forum for awhile, and read all of the FAQs, before making your first post. Get a feel for the currents.

Maybe the worry is that, if its not also designated a safe-space, the control/direction of the group will be shifted away from the hands of the people belonging to that group, over time, just by sheer mass/inertia from the dominant group? I don't know.