Sakazuki is a good guy, for crying out loud ace was a pirate, they plunder people. Marines are not the bad guy so I see no reason for Luffy to fight the marines, the only scenario that the strawhat fight the marines is if the marines support the world government, and there would be someone stronger than akaunu who would be reserved for luffy, while akainu might be reserved for sabo because of the magma vs fire stuff