Why? Is a couple not allowed to have a 'domestic' on an aeroplane at all? This was probably a little extreme, but the crew were right to be muted in their response. Some couples habitually behave like this. Deep down, they need it emotionally. When someone interferes, they can both turn quite savagely on the peacemaker who is then totally bewildered, and then becomes 'the victim', much to their astonishment. Right to move the victim of the couple elsewhere if they want that, but that cannot always be done. But if the crew went for restraints on the aggressor, they would find the victim may become quite savage in the protective response for the aggressor. I'm afraid the crew are in a cleft stick, whatever their response, some people will find it inadequate or incorrect. They both bought FC seats, and want to behave like that in public.....let them get on with it! As long as nobody gets hurt, and the victim of the pair is not asking for assistance or help, how they behave is up to them, within the constraints of the law and travelling with other people in close confines.