This has probably been overdone to the moon and back times ten, but what the heck, let's do it again.
The New Testament seems pretty vague on its sexual ordinances. It forbids adultery and prostitution, and Christ condemns us from looking at each other with sexual intent, but then you have St. Paul using blanket and somewhat relative terms like "sexual immorality" or "fornication," both of which are translated from pornea, which has several definitions.
That being said, how does one define "sexual immorality" as different cultures have different views? As has been said several times--and quite controversially so--the New Testament never condemns polygamy, likewise, it never condemns sex outside of marriage (AFAIK), which, is one of my problems with Bibles that use the term "fornication," as the word usually means pre-marital sex, even though pornea has several meanings.
That being said, what does the Patristic tradition tell us? What is the earliest definitive source we have on it? Is there perhaps a Canon or quote by the ANFs defining what is sexually immoral and what is not? Does the Didache mention it? What is the first Christian source to explicitly state that all sexual relations outside of a monogamous heterosexual marriage are immoral? How did the Fathers understand "pornea"?