The forum's software has, in your Profile, the option to select if you are a male or female. These are the only two options.
Registered members are allowed access to the Private Lives board. If men were allowed to join, the only way to disallow access to that board would be via the select option in the profile.
There's nothing to stop a man selecting 'female' and then accessing the board.
Women who have been posting for years on the understanding that the Private Lives board is for women only, would be frantically deleting their posts and accounts ruining the forum. Also the legal aspect may have to be considered. If you've agreed to join a women-only forum and find out later that men have been allowed access to your private posts, that's a breach of trust.