To me, Quina is a distinctly feminine name, with the final "A". It's just how my italian brain is trained to recognize names. (My language also marks feminine/masculine, and Quina is always addressed as female in game.)

Plus, she... wears a dress. Pink. With frills. Now I know Kuja's clothing is probably even more feminine... but if you want an asexual effect, you don't go with a skirt, pink and frills, and a distinctive feminine sounding name.

I think the English translation pushed it with all the s/he (which looks VERY stupid - honestly? They're supposed to be talking. How do you pronounce S/HE? Don't you just end up pronouncing a "she" sound?) in the jap version, Quina just didn't have a specially distinct feminine/masculine way of speaking, but big deal! - many people don't. It doesn't mean they don't have a gender.