I only ever used the girl in Crystal, because she was new and she looked cool
Ever since then I've played as a boy
Also, at the time I picked the girl for Crystal because I used the default color name for all the games before and unlike Red/Blue/(okay I used Ash for Yellow XD)/Gold/Silver, Crystal is a girls name.
I didn't really care for it though and didn't continue the tradition. No, wait! I used Green for Leaf Green just to complete the original trinity, but I started using a moniker of my choosing with everything else including Fire Red.