Thankfully, you're wrong.

If that were the case ShieldsUp, over at grc.com would not show currently forwarded ports, but not being used ports, as stealth. Would in fact show those ports as open. Test for yourself if you wish.
For example if I have port 80 forwarded, traffic is only allowed in when I'm actively running a web server. Once I turn off the web server the router, the firewall, blocks it. The forwarded port is no longer valid.
Forwarding ports only become active once data has already been sent out those ports. Or an application on your system, within your network, is actively running on that port.
This is true, because we are running NAT routers. Port forwarding is not for opening wholes in the firewall per say, it's for translating private addresses into public addresses, and vice-versa, or NAT traversal if you will.
Basically the same as UPNP, only UPnP does it automatically for you. I personally just like more control over it.