Good points. Would you agree with me, however, that up to about the age of 12 or so, there is very little difference, if any at all, between the sexes when it comes to sports? Fast girls are just as fast as fast boys, strong girls as strong as strong boys, etc. It's only when puberty kicks in that the differences emerge? In my experience this is true.
(In my 20s, I was employed as a coach for a large Iowa school district, and I have experience as a coach with junior high basketball (boys and girls), junior high and high school volleyball (girls), junior high track (boys and girls) and high school football.)
I'm not sure I'm a good judge of that because my experiences are affected by the fact that when I went to elementary school there was a big difference....but that might have been due to other influences (as you suggested).
Today when I watch my 12 yr old daughter compete in soccer (and at a younger age it was co-rec...as was basketball), there didn't seem to be a huge disparity between boys and girls (although boys tended to me more aggressive) athletically.