Crimes against nature
A. If any person carnally knows in any manner any brute animal, or carnally knows any male or female person by the anus or by or with the mouth, or voluntarily submits to such carnal knowledge, he or she shall be guilty of a Class 6 felony, except as provided in subsection B.
B. Any person who carnally knows by the anus or by or with the mouth his daughter or granddaughter, son or grandson, brother or sister, or father or mother shall be guilty of a Class 5 felony. However, if a parent or grandparent commits any such act with his child or grandchild and such child or grandchild is at least thirteen but less than eighteen years of age at the time of the offense, such parent or grandparent shall be guilty of a Class 3 felony.