I agree with SH8!
How many boys can do splits, compared to girls? How many women have lots of flexibility in their hips compared to men?
The ideal leg/hip shape for riding a horse would be an inverted you. Most men I know - and certainly me - are an "A". That tightness in the hips makes it very hard to settle into the saddle. My wife, daughters and daughter-in-law all settled into a saddle pretty much the first time. It has taken me years for my hips to loosen enough, and even then I need 10-15 minutes of easy riding before I am comfortable with trotting/canter.
And don't get me started on western saddles that rise sharply from the cantle forward...those were NOT designed by or for men! In fact, they have to be designed by women who have had an ugly divorce or two behind them...