Why didn't Boa Hancock's attack affect Luffy? Why didn't Boa Hancocks attack convert Luffy to stone?
It didn't look like he used Haki or something which would nullify the effect!
Is there any possible explanation for it?
From the One Piece wiki page on Mero Mero no Mi (Hancock's Devil's Fruit):


  The Mero Mero no Mi is a Paramecia-type Devil Fruit that allows a range of attacks which use emotions of lust or perversion to transform opponents into stone. 


Luffy did not have any lustful feelings towards Hancock, so her Mero Mero Mellow attack had no effect on him.