Paddy,
It's not unknown on a Friday night for Club Europe to be full of men and no women travelling back from Europe to Heathrow. Now if a UM was booked in Club Europe with an onward connection to Oz, then what? Downgrade the UM, and upgrade someone from Economy? You can see that going down well with the UM, espeiclaly if they are expecting a meal. Even if every possible opportunity was taken at check in to avoid the problem, one can see that circumstances can arise from this rule where CC are in deep dodo whatever they do - especially if there's been cancellations.
I've had a similar experience on Amercian Airline. The CC wanted to keep the last row clear to put stuff on, and lady with three kids had them in the same row as me, and her over the aisle. CC didn't want to know (usual AA CC bloody mindedness) until I politely asked if they didn't have SOPs for unaccompanied males sitting next to children. With bad grace, she moved me back a row......probably because she figured someone knowing what SOP stood for just might be able to cause trouble.