Setting it to private would limit its discoverability for people who are welcome to contribute, which as you say, is about half of all people. Some people like to lurk and read without posting which is perfectly fine and even welcomed. It’s not a private forum, it’s a forum that just asks men not to post.
In terms of how forums work, it is saying it wants to be private but also public. There is no default setting for that, but there are ways to do it that achieve what you want without breaking the public/private system. For instance you can make the forum public, but set it to only allow comments from approved users.
Setting it to private would limit its discoverability for people who are welcome to contribute, which as you say, is about half of all people. Some people like to lurk and read without posting which is perfectly fine and even welcomed. It’s not a private forum, it’s a forum that just asks men not to post.
In terms of how forums work, it is saying it wants to be private but also public. There is no default setting for that, but there are ways to do it that achieve what you want without breaking the public/private system. For instance you can make the forum public, but set it to only allow comments from approved users.