To be honest, despite tons of fanart I've seen and various fanfics I'm sure are out there, I've never actually seen anything in the One Piece manga or anime to suggest pirates Luffy and Nami are a couple. True Nami sometimes exhibits tsundere habits such as yelling at or punching Luffy, but this is mostly due to him saying or doing something that gets on her nerves. Still, Nami was the first person Luffy met who would become member of his pirate crew and there have been times where Luffy has been implied to find Nami attractive.