muuu: peky is known for being quite hard to understand, but I can hazard a guess here. Translation to normal language:
"The reason is that in cis-females, no matter the actual width of the pelvis, femurs are inserted in the pelvis with an angle towards the inside, compared to males."
In short, women naturally have their knees closer to each other, as well as the other end of the femur farther from the center of their body, so it's less effort to cross their legs.
As for me, I think one of the main factors for my difficulties in crossing my legs is that my legs' muscles are HUGE and get in the way, working in splendid teamwork with the fat there. Combine that to my sorta-wide-for-a-male-but-definitely-not-female hips, and there you have the difficulty already. And I suppose the thing about the femur's angle is true too.
Though unless you're super obese or have zero flexibility, crossing your legs relatively comfortably should be possible for everyone, even if you need to pull your leg over the other with your hands a little. Many guys do it. Just gotta make sure you don't put your genitalia just in the wrong spot, because obviously there might be a problem.
AllieM:

MaidOfOrleans: Seriously, I'm pretty sure one would notice right away if it were that, and not even ask. I mean, there's a huge difference between "ugh, it's hard to reach" and "eek, my junk is being squeezed!" Besides, junk-squeezing doesn't prevent anything physically, does it? It just makes it more painful, I guess. Dunno, I've never really managed to get hurt down there. Either mine are much less sensitive, either they're evasive to blows and squeezing. :p