It sounds like the two factors are the junk as well as the difference between male/female bone structure. My guess is it's a combo of the two, and the amount that each one contributes varies from person to person.
Personally, I've found that it's much easier to cross my legs when I'm tucked. I found that my legs just don't quite want to cross that way when untucked, even though it doesn't feel like the junk is in the way.
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