It depends on what the character looks like or if I played that gender for way too long. Like for FR/LG. I always played as Red, but now I just play as Green not because the way she looks, but because I got tired of looking at my Red sprite for quite a long after multiple playthroughs. G/S/C, I'm mixed, I want to play as Gold, but I don't like the orange/red colored sprite so I pick Kris just for the blue colored sprite. R/S/E. I never picked May, I don't like the way her hair looks when she throws the Pokeball so I always go with Brandon. D/P/Pl, I pick Dawn because I don't like the way the male character looks. B/W, I mixed them up because I like both designs, but mostly i'll go with Hilda because her throw is amazing ( ͡° ͜ʖ ͡°). B2/W2. Hmm, ehhh... huh...
And for X/Y, I'm fine with both but more over Serena than Calem. But that's just me.
Edit: Forgot about ORAS, I pick May over Brandon now. So the majority is i'll pick over the females than the males.