Consider a system with one developed planet with a starport nearby. I get surprised by the arrival of an enemy fleet of two ships and a transport. I also have two defensive ships in the system of approximately equal quality, but the enemy ships are outside my range, so I decide to keep them defensive. I'm unable to rush more ships. Unfortunately I have a choice of two poor options to make because the enemy gets to choose his target. 1. Keep the ships in planetary orbit, but I lose my starport, or 2. have my ships defend the starport, but I get invaded. To properly defend both planet and starport, I need 4 ships. That's unbalanced in favor of the aggressor. It's also not a problem in GalCiv2 where the shipyard is on the planet.
What mechanic am I missing that solves this issue?