Jason BeDuhn has made some cogent and generally objective statements regarding the translation of a difficult passage in the NT.
1 "Fornication" is a word that has had its day, and the expression "sexual immorality" is far more intelligible to the growing number of youth who may take to a reading of the Scriptures. Most modern English langiage translations use this expression.
The RSV of 1951 has "unchastity" Some translations strike a moralistic tone: "sexual vice" [Moffat] "sexual sin" [Expanded NT- pub in 2009] The original Berkeley NT [translated by the faculty of Bible Languages of UCLA, Berkley, California under the guidance of Prof Gerrit Verkuyl] of 1945 has "lewdness" but a later revision of 1969 reverted to the the classic "sexual immorality" Other unusual translations: "prostitution" Concordant Lit NT, "Whoredom" Campbell NT of 1951.
2 The way to render verse 5 in particular provides the translator with some distinct problems. This is because Paul here uses two separate words, almost overlapping in meaning, both of them being nouns. "Pathos" means "passion" as a noun, and "epithumia" means, well, passion as well, although to distinguish it from the first word most translators use "lust". So Paul is literally saying, if you take the words as nouns with the second a genetive: "passion of lust"
Rather than render both nouns as nouns, most translators indulge in some sleight of hand, to make the sense more intelligible. Some take the first word as an adjective, as does NIV "passionate lust", the NASV on the other hand reverses the order and has "lustful passion" The Roman Catholic NAB has "passionate desire"
Some translations do preserve both nouns with such renderings as: "passion of lust" ESV, the RSV, and the Roman Catholic Confraternity NT.
The problem is compounded when we consider that "epithumia" translated here as "lust" by most moderns, does not always have an evil meaning. The sense that undergirds this word is one of deep conviction, and may indeed have a beatific significance. Jesus for instance used this word to define his "eager desire" [NIV] to eat the Last Supper with His disciples.
Paul used the word twice, once to describe his "desire" [NIV] to be with Christ on his death [Phil 1:23], and, as well, to see the saints of Thessalonica again [1 Thess 2:17]. However here, two chapters later, it is obvious from the context that there is a suggestion of evil intent present.
3 The use of "nation" for "ethos" is a vexing one, and it depends on what the translator is attempting to do. Either one puts the word into a modern socio-political context, or one preserves the original sense of being that sustained the values of the NT world. One can't unfortunately, do both.
The word "nations" may sound "reasonable" and politically correct, but does not have relevance to the milieu in which the NT was crafted. It is to read into the NT a reflective modern contextual grid. There were no nations in the then known world. Brittania was not a nation, neither were Greece, Gaul, Germania, Egypt, Judea, Asia Minor etc. The Pax Romana was supreme over all, and all were merely provinces within a Super-state.
Besides that, to be brutally frank, the NT does make a distinction between two ethnic groups, The Jews and Gentiles. Paul often distinguishes between the two [Acts 13:46] and even between Christians and Gentiles as here at 1 Th 4:5. While there was an instinctive attitude of prejudice among the Jews, which sometimes, unfortunately carried over in the Fellowship of Believers, as in the case of Peter and those called "Judaisers", Paul strived to show that the two divisions of humanity recognized by the Bible were to be bonded forever into One Fellowship. This was made possible, Paul says in Eph 2:14, by the blood of Christ "Who destroyed the barrier, the dividing wall of hostility" Now the Gentiles, as Christ's other sheep were to be one flock, with their Jewish counterparts.
Thus the enduring work of Christianity which vaulted over prejudice, and made possible a sublime unity of division within a world of prejudice, is actually lost if the translator doesn't alert the reader to this basic pattern of subliminal apartheid.