Just as a matter of interest. Would that male passenger have been asked to move had he paid BA's seat levy for the location he occupied?
You are making the assumption that the rule states that "male passengers should be moved"!
The fact of the matter is that the rule states that UM's cannot be seated next to "Male passengers travelling alone"! Therefore it is actually up to the checkin agent to ensure when the UM is being allocated a seat that it is next to a female. What happened on board this flight happened because the crew member WAS IN THE WRONG. This man was NOT travelling alone so therefore does not come under the ruling AND even if he was, the rule does not state that the male needs to move, the crew member should have used some cop on and just move the kid without causing any fuss or even needing to reveal the rule.
Every company has idiots like that who both get the rules wrong and seem to glee in imposing their version of the rules on others...suppose its a power trip thing!