"Portae" means "gates" in Latin, and ports (like seaports) can be thought of as specific places in a wall or boundary where things or people go in or out. GeraldWeinberg in "An Introduction to General Systems Thinking"(1975), describes a port as
... a special place on the boundary through which input and output flow... Only within the location of the port can the dangerous processes of input and output take place, and by so localizing these processes, special mechanisms may be brought to bear on the special problems of input and output.
R.M. Balzer also wrote "PORTS - A Method for Dynamic Interprogram Communication and Job Control" in 1971, according to him a paper that was much cited, but never really led to anything! A pity!
Now doors have an "inside" aspect and an "outside" aspect - the name of the inside aspect might be used by people inside the house to refer to doors, e.g. "let the cat out the side door", while the outside aspect is related to what the door opens onto, e.g. back garden of the house at 12 Main St., and will be of interest to city planners or visitors. Ports in FBP have the same sort of dual function: they allow an FBP component to refer to them without needing to be aware of what they open onto. Port names establish a relationship between the receives and sends inside the program and program structure information defined outside the component.