I sit with my legs wide open. lol. That's why I don't wear dresses or skirts, well that among other reasons. It's comfortable, feels natural etc. But I also do it as a little rebelling thing towards my dad. He made me attend these f***ing etiquette classes when I was a kid, and one of the things we covered was how to sit properly. Because I'm a "lady", I have to sit properly and primly like I have a f***ing steel rod rammed up my ass and spine.
The reason why guys do it though is pretty obvious. They have a penis and testicles between their legs. Also, it is scientifically proven that the wider a males legs are apart, the more dominant his personality and the larger his penis. Because I sit with my legs wide open my friends always crack a joke that I have a one-hundred foot long penis. Impressive, eh?