What is the teaching of the Holy Roman Catholic Church?
Catholic teaching of this matter is given to us by God Himself
in the Bible:
heed to keep thyself, my son, from all fornication: and
besides thy wife never endure to know a crime." (Tobias
shall be no whore among the daughters of Israel, nor whore-monger
among the sons of Israel. (Deut. XXIII
you abstain from things sacrificed to idols and from blood
and from things strangles and from fornicaton: from which
things keeping yourselves, you shall do well. Fare ye well.
(Act. XV 29)