OK....so lets try to reach some sort of agreement
Sport per se should NOT be mandatory. BUT.....
EVERYONE in the forces (be they RAF pilot, Navy diver or Army chef) has to be fit. after all, there is no such thing as a "non combatant" serviceman/woman. In the absolute worst case scenario EVERYONE has to be able to pick up a weapon and FIGHT.
Hence the need for fitness. Also being fit will help you cope with the rigours of service life. Most members of the frorces have stressful jobs....something that regular exercise will help them deal with.