When I first started playing, (like 2 years ago or so) I only used males.
Then, that changed and I would only try and get females and be somewhat disappointed if the poke I got was male. Nowadays, that's still KINDA in effect but to a much, much lesser extent. I just tend to prefer pokes female for whatever reason. I do think some pokemon look very male, while some look very female (jigglypuff, gothorita, chikorita, machamp)
Im not sure if anyone would agree with me but I find that Male pokemon are stronger but slower and females have better defense and they seem to be faster regardless of nature. I dont have any proof to detail but it just seems like that to me.
When I was a little kid (Gen 2-3) I could never bring myself to catch and train female pokemon, always just thought guys were tougher. Now I've grown up a bit, and just use certain pokemon only as females (like Garbodor or Chansey) because their better competitively (namely at things like taking out the trash or rearing children) XD