Hi MissKairi. Seems like this and a lot of other stuff is just learned behaviour, as the others have said. Perhaps I've misunderstood, but it sounds like you're finding it difficult to sit that way? When I was starting a few weeks ago, it bothered me that women often sit with their legs crossed and I couldn't do it without squashing my bollocks uncomfortably. So, I started out with an easy-to-remember rule: don't cross your legs, but always, always keep your knees together when sitting down. You can spread your feet out a bit to make it easier. If you stick to that, and keep your elbows in, you can't go far wrong. (That said, I'm still getting male-gendered all the time, so maybe I'm talking rubbish.) After a week or two of this rule, I realised my, uh, man bits were getting kind of used to being a bit squashed between my legs, and I was able to cross my legs (still only one way, though) for ten minutes or so without a problem. Wearing tighter underwear has helped - however you sit, you're still squashing stuff down there, so you might as well sit like a lady.
It seems to have something to do with weight, though. I'm a fairly standard guy weight, but if I were thinner, I think I'd be able to cross my legs for longer. Anyway, sorry for the TMI and rambling, hopefully this helps.