IMO, this problem is mostly based on the art and design team. I could believe that almost all their team is composed of males. While I may be wrong, I really don't know many females designers.
While one could learn not to, most males (I believe all of them might agree) tend to create an sexy female image right from the bat when you do have the option. Meanwhile, it's rather weird for a straight guy to be told "create a male image" and make it sexy from the start. I could guess that is because women pay attention to female beauties, while men tend to ignore male ones. In my case for example, I could name a stupidly long list of gorgeous women, but as for men... perhaps a list of 10 names that a I hear my female friends and related females say that are good-looking, like a few actors and sportsmen. Prolly it's the same for most guys and beng in the design team, one would first need to change that point-of-view, changing the fixed idea about the Cool(in a hero or villain way) or Funny guys and Sexy girls, which is (at least IMO) the natural course of thinking a male designer would follow.
As a sol-ution, perhaps Riot could employ more female designers or ask for more opinions from their female employees, even if they are not related to art and design, possibly asking them what is this guy missing? Not really hard I think.