Don't Cross the Streams

On the deserted Kokku Island, the Straw Hats (save Chopper) all drink the milk of pink coconuts. When they wake up the next morning, Nami and Robin have become futanari (girls with certain male parts) and Luffy, Zoro, Sanji, and Usopp have turned into girl versions of themselves! Unaware if the changes are permanent, they have fun with their new bodies while they can. Futa, yuri.