Swords are the soul of a swordsman. For Zoro, his swords are very precious to him, especially Wado Ichimonji; but Nami, not only kicking and touching but also asking to use his sword (in anime). Apparently, he lets her do that, when Zoro was in danger, Nami helps him immediately by kicking his swords towards him, letting he use 100% of his power.
When she asks to use his sword, he doesn’t feel uncomfortable or refused, he just reminds her not to kill the child, and after that being excited seeing Nami using his sword, not even wanted to stop her (he knows she wouldn’t do anything harmful).
In One Piece Z official art, Oda draws Nami touching Zoro’s Wado Ichimonjo, his favorite sword, they are close enough to feel free for touching other’s things.