I present to you the following evidence (in bulleted format and behind an LJ cut for your convinience)
Why Luffy is not a ninja
His goal is to become the Pirate King, not the Ninja King.He does not practice any form of ninjutsu whatsoeverHe could be hardly beconsidered stealthy by any definition of the word\
His mentor is a pirate. (Remember Shanks? Definitely a pirate!)His idol was the Pirate King! (Gold Roger... no doubt there... PIRATE!)Why Sanji is not a ninja
He cares too much about cooking and pretty ladies to even care about being a ninjaHis boss at Baraties was a former pirateWhy Zoro is not a ninja
His pirate hunting days where simply a way of making money, not out of any particular malice toward pirates.He is the first one to join Luffy's crew.Why Nami is not a ninja
This can be summed up in one tiny little phrase: She loves treasure!Why Usopp is not a ninja
His dad is a pirate.His goal is to become a great pirate like his fatherHe had his own mini-pirate crew in his hometownHis desire to be called Captain Usopp. I hope that serves in correcting your mistaken notions, you pajama wearing, bacon-less ninja. :P