Because it is comfortable and conveys emotions. If you are not uncomfortable and push those emotions down till you reach the point of critical hear failure, you are not acting masculine.
Not sure why you brought up masculinity, though — I'd argue that getting your balls cut off doesn't make you any less of a man. It certainly does expand your repertoire of safe and comfortable seating positions, though.
No, I meant crossing your legs won't thin your masculinity like I said. I brought masculinity into this because the person I responded to had already interjected it with the castration statement.