I play as a boy because not only am I one but I actually like to have a character that is as close as it can to me. Just to make the overall playthrough better, I guess. I simply imagine myself as the player better when I play as the male character because I obviously am one.
I used to play half of the time as the girl, depending on whether I prefered the boy or the girl's outfit and design. For example, I think Leaf, May, Dawn, Rosa and Serena look better than their male counterparts. I ended up playing as May and Dawn (in one of my Black playthrough as Hilda as well) in my Sapphire and Diamond versions. But one day I restarted all my main games and I set myself on one specific gender; my real one. Ever since that day, I learned to appreciate the boy characters and look at them in depths. I'm a real fan of Brendan and Lucas now, but my opinion on Red and Nate stays neutral. I really dislike Callem's design as well, but the way mine is customized in-game makes it really awesome.
Plus, I prefer when the female counterpart is your rival, in cases where it happens. Even if I was playing as the girl, I felt like a boy inside with only male rivals and I felt the lack of feminity in the relationships I had between my rivals.