The Literal Commandment
You shall not uncover the nakedness of your father, that is, the nakedness of your mother. She is your mother; you are not to uncover her nakedness. (Leviticus 18:7)
The literal commandment is that a person must not "uncover the nakedness" of your father, that is, the "nakedness" of your mother.
"To uncover the nakedness of" is a Biblical idiom for "having sexual relations with".