This is being asked in he context of a router that has no port forwarding enabled. I.e. no incoming connections accepted. Outgoing only.
I read something on a forum where someone said that whenever you as a client are behind a router, and connect to a particular host, then that opens up a port on your router and forwards ALL inbound traffic attempting to connect to the port, regardless of the source of that traffic, and thus while that connection is open it is just as insecure as if you setup port forwarding for that port. In other words, some malicious third party, that is coming from some other address besides the host, could connect to that port.
My understanding was that when I initiate an outgoing connection to a host, then the listening port is limited to communication with that host address. In otherwords, a third party's incoming connection would not be able to access that port. However, this one lone person seemed to imply that only a firewall did this. This kind of violates my understanding of how a connections works and how a router maps the listening port for an outgoing connection(I was going to add there's plenty of holes in my knowledge to violate, but then I realized that sounded really bad.).
Does a router limit an outgoing connection to communicating with the destination host? Or does the router also forward from other third parties that are not originating from the destination host if they try to connect to that listening port?