Don't see the problem .... I've always seen this forum as a Happy Hour bar on a flying station (I know I've said this before) with a number of conversations going on. Lots are aircrew, and quite a few aren't. Similarly, the subjects may or may not be closely related to military aviation.
So what - your presence here and your contributions are both voluntary ... I'm sure I could go into a Happy Hour conversation and say "This is a boring/irrelevant/insular conversation ... please leave". And what a
pr@t I'd be. I just don't join that particular conversation.
Nor do I say "You're not in uniform - b*gger off" ... 'cos believe it or not, filthy civvies may have something useful to contribute. And/or may be retired or wannabees, both of whom IMHO have a perfect right to hear and be heard - in the bar or on pprune.
As to a closed forum - the military ain't BA - just consider how you would "vet" people and what the security implications might be....
Unnecessary in principle - unworkable in practice.