Just my opinion here.
If a forum publicly states that it is not a free speech forum, but designed for those of a particular view point. Then Tuff noodles for anyone who does not like that. Was it OK to force the Boy Scouts to accept girls among other things? This is no different. Everyone has the right to form groups consisting of people they agree with, and for a purpose.
If a forum says they are about free speech(Twitter) then that is different. But even that gets complicated. Do they have to allow illegal content? Porn in all settings? Violence?