It's kind of sad that we can't have a mature discussion about sexualisation on the boards, at least not without so many comments missing the point.
Anyway, if I understand you correctly your main complaint is not that female champions are sexy, but rather the unrealistic nature of their splash arts. Wrong body proportions, spine-breaking poses, impractical armor and so on. Additionally, the splash arts focus too much on displaying that sexiness when they could be used to better emphasize the character of the champion.
While this is certainly true, the core issue lies not with Riot Games, but with media and human psychology in general. Our minds don't really understand how human bodies should work until we start thinking about it. Sex sells, so everybody started using sex to sell. We have been fed unrealistic body proportions through the media for years, to the point where the ideal body marketed to us now is borderline anorexic. Normal body proportions look fat in comparison, normal poses look like bad posture. Sadly, marketing with realism is just less effective.
While I would really appreciate Riot trying to change the general view of how a realistic attractive body should look like, they can't do it by themselves and would more likely than not reduce their income through that decision.
However, what they can do is settle on compromises. Riot already has made progress. We have less sexualised characters lately, sure a few spine-breakers here and there, some wide shoulders sprinkled in between, but ever since Vi's release there has been a drastic change in champion designs.
As long as Riot continues this trend, I'm fine with some releases like Kai'Sa or Braum from time to time.