Well... The way I make sense of it, and that's just my own conjecture, is that Sanji was very sensitive after a 2 year draught. Then Chopper started rehab and was trying to slowly get him used to women again, and I think that was working, because Sanji was also seeing lots of them again (like out on the streets in Sabaody) and spending time with Nami and Robin, and it seemed like he was building up at least some resistance, like, he could hold it together a little while before getting overwhelmed. And then he went swimming with mermaids, which was a dream of his, and that overexposure did the job, you know, how you try to cure phobias via exposure? (almost killed him anyway, though) Or getting a blood transfusion from Okamas insta-cured him, because he never wanted to have to go through that again (or maybe their blood is just supposed to be resistant to women, idk). Either way, him turning to stone is the signal that he is returning to normal, because that's also what happened when he saw Hancock, the other world famous beauty. Hancock wasn't using her powers on him, so it was just a visual gag that happened again here. And a good thing too, I was starting to get real tired of the joke again, sigh.