Ok, there are many who never got smothered by an ass so, let try to explain whats up down there. At the beginning everything is pleasure. When you get the chin buried between the buttocks theres no chance to use the mouth to breathe. But its ok, if the girl let you, we still be able to get some oxigen thru her panties using the nose. But few minutes later the jaw starts to hurt cos the girl weight. The heat and sweat down there become a problem and now, of every goes fine, we will see white spots but, believe it or not well feel a great boner. This is normal unless you were not a freak XD
