The word used in the passage is TAZNUTH.
It is defined (as is PORNOS in the Greek) as "unlawful sex."
So, that verse doesn't get us there.
We still need to know what makes it "unlawful."
Exodus 20:12
Honour thy father and thy mother, that thou mayest be longlived upon the land which the Lord thy God will give thee.
Seems to me that not wanting his parents know is hugely unlawful in the OT.