Sec. 16-23. Adultery and fornication by persons forbidden to marry.

If any person commits adultery or fornication with any person whom he is forbidden by law to marry, such person shall be guilty of a Class 1 misdemeanor; provided, however, that this section shall not be construed to apply to a person committing adultery or fornication with his daughter or granddaughter, or with her son or grandson, or her father or his mother.