From the man page:
 -g      Allows remote hosts to connect to local forwarded ports.

When working with a control socket, this works fine if -g is included with the initial connect attempt, for example:

$ cat ~/.ssh/config
Host *
ControlMaster		auto
ControlPath		~/.ssh/%r@%h:%p

$ ssh -gTfNL 12345:localhost:12345 host
$ netstat -tln

tcp        0      0 0.0.0.0:12345             0.0.0.0:*               LISTEN      -

$ lsof -n

ssh       182446        user    3u     IPv4           76397177       0t0      TCP 192.168.0.9:51181->192.168.0.15:ssh (ESTABLISHED)
ssh       182446        user    4u     IPv4           76397181       0t0      TCP *:3128 (LISTEN)
ssh       182446        user    5u     IPv6           76397182       0t0      TCP *:3128 (LISTEN)

and similarly:

$ ssh -g host
user@host:~$ exit
$ ssh -gTfNL 12345:localhost:12345 host
$ netstat -tln

tcp        0      0 0.0.0.0:12345             0.0.0.0:*               LISTEN      -

However, if it isn't then -g is ignored on subsequent attempts to forward ports:

$ ssh host
user@host:~$ exit
$ ssh -g -L 12345:localhost:12345 host
$ netstat -tln

tcp        0      0 127.0.0.1:12345             0.0.0.0:*               LISTEN      -

$ lsof -n

ssh       182399        user    3u     IPv4           76390396       0t0      TCP 192.168.0.9:51178->192.168.0.15:ssh (ESTABLISHED)
ssh       182399        user    4u     unix 0x0000000000000000       0t0 76390976 /home/user/.ssh/user@host
ssh       182399        user    6u     IPv6           76392394       0t0      TCP [::1]:3128 (LISTEN)
ssh       182399        user    7u     IPv4           76392395       0t0      TCP 127.0.0.1:3128 (LISTEN)

This doesn't really make sense: ssh should still be capable of binding to the correct address as requested.

From:
https://bugs.launchpad.net/debian/+source/openssh/+bug/1259939
http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=731950