Not for uncleanness, but in sanctification (epi akaqarsiai all en agiasmwi). Sharp contrast made still sharper by the two prepositions epi (on the basis of) and en (in the sphere of). God has "called" us all for a decent sex life consonant with his aims and purposes. It was necessary for Paul to place this lofty ideal before the Thessalonian Christians living in a pagan world. It is equally important now.