Weight & Balance (Trim) are not relevant once airborne
I seriously hope you do not really believe that. Aircraft trim and balance are relevant through the entire flight envelope. There is no law of physics that mysteriously disappears when an aircraft takes off. The same system used to trim the aircraft in flight is used by the pilots to trim the aircraft for takeoff. A trim system is used to relieve control loading, that loading may be held by brute force but is tiring. There is a point at which no amount of brute force (or a trim system) can hold enough control deflection to allow safe flight. In flight the pilots (or autopilot) will use this trim system to maintain light control loading, but the laws of physics will dictate when stability is lost.
If you were to load most modern jet aircraft with passengers and freight only at the rear of the aircraft and none at the front, they would not fly safely - they would be out of trim. This would not change magically if you could somehow take-off.
You are entitled to an explanation, and courtesy - and then the entitlement thing stops and you are required to comply. Your 'comfort' or seat preference are factors, and ones that a good crew will manage appropriately - but they are irrelevant when given a clear instruction by a crew member, acting with the delegated responsibility of the Captain (as they are). I'm afraid this one isn't up for discussion.
This is on the money, if issued an instruction regarding the safety of the flight by the crew, failure to comply is a criminal act under the ANO. Crew do use the old trim argument to move people as it is easier to baffle people with a 'spurious' technical reason than tell them its for reasons of exit coverage etc.