Why did Luffy react to Nami's body, but not to Hancock's? When Luffy saw Nami's naked body thanks to Mr. 2 and later in chapter 213 due to her happiness punch, he reacted and got excited. But on the other hand, when Luffy saw Boa Hancock naked it did not affect him in any way. Why was Luffy affected by Nami's body, but not by Hancock's body?
A different reader had the same question and according to Oda, it was all because of Usopp's bad influence. Luffy and Usopp are of the same age, so  when they are together it would be like kids going on a school trip: their bad side comes out!
From SBS of Volume 54, chapter 523:

Reader: Hey, Oda. In volume 53, chapter 518, Hancock's &quot;Mero Mero Mellow&quot; had no effect on Luffy at all, but in volume 23, chapter 213, it looked like he had a reaction to Nami's &quot;Happiness Punch&quot;. Why did he respond to Nami's naked body but no to Hancock's? Is Nami's body just that amazing? -Kazu
Oda: Wow, I got a lot of questions about this. Is this what you guys are paying attention to? Don't just read manga all day, go study some more!! Okay? Well, I say that, but I do nothing but draw manga. I noticed this when we were drawing it, but if Luffy had a nosebleed when he saw Hancock's body, I think you would all be like &quot;Ehh!?&quot;. That's not the Luffy we know. I believe that Luffy reacted to Nami's naked body twice, in volumes 18 and 23, and both times it happened, Usopp was with him! He's the suspicious one! In other words, when Luffy is alone, his reaction is what it was with Hancock. He's interested, but he's not entranced by her. But when he's with Usopp, who's the same age, it's like a kid on a school trip: his bad side comes out!! Yeah, both sides of Luffy feel right to me, so the culprit is definitely Usopp!!