I think you're underestimating how hard it is. Every single major sporting event - football, basketball, baseball, hockey, golf, tennis - has athletes not acting like adults under your meaning. Even Roger Federer, the poster boy for niceness, curses loudly. Hell, you've just spent several hundred words arguing that players actions on the field make FANS act immaturely. So this idea that players should act in a superficial grown up way is, again, absurd. Players do act like grown ups. They just act like grown ups in a much more competitive and important job than yours. Swearing, barking, staring down opponents - all things you've associated with immature acts - are normal and correct behaviors.