A. Any person who engages in sexual intercourse with any person whom he is forbidden by law to marry is guilty of a Class 1 misdemeanor except as provided by subsection B.
B. Any person who engages in sexual intercourse with his daughter or granddaughter, son or grandson, or father or mother is guilty of a Class 5 felony. However, if a parent or grandparent engages in sexual intercourse with his child or grandchild, and such child or grandchild is at least 13 years of age but less than 18 years of age at the time of the offense, such parent or grandparent is guilty of a Class 3 felony.
C. For the purposes of this section, parent includes stepparent, grandparent includes step-grandparent, child includes a stepchild, and grandchild includes a step-grandchild.