Romantic moments for Luffy (and other Straw Hats)
Actually there is a canon answer... from the Word of God himself... even if it doesn't help.

D: Doesn't anyone on Luffy's crew fall in love? Will there never be a romance among crew members? (Sanji is an exception in this case) I'm just wondering. from Naoko 

O: Of course they're in love... WITH ADVENTURE! (good one)

http://www.japanator.com/eiichiro-oda-answers-fan-questions-in-sbs-7443.phtml