In many cultures throughout the world, crossing one's legs is taboo and considered very disrespectful. In North America there are no real taboos against such action, rather, we tend to cross our legs to get comfortable. Should we do so in church? No. Not because it is “wrong” for us ever to cross our legs, but because it is deemed too casual—too relaxed—for being in the presence of God.
When we get settled in our favorite chair at home, we lean back, kick up our legs, and allow our minds to wander. Remember, sitting in church is a concession, not the norm of prayer. We strive to remain attentive (i.e.: “Let us attend”) at all times as a soldier prepared for (spiritual) battle before his commander. Should we sit, we do so attentively, that our minds not wander from the “one thing necessary.”