Adam Clarke on 1 Corinthians 6:18

Flee fornication - Abominate, detest, and escape from every kind of uncleanness. Some sins, or solicitations to sin, may be reasoned with; in the above cases, if you parley you are undone; reason not, but Fly!

Sinneth against his own body - Though sin of every species has a tendency to destroy life, yet none are so mortal as those to which the apostle refers; they strike immediately at the basis of the constitution. By the just judgment of God, all these irregular and sinful connections are married to death. Neither prostitutes, whoremongers, nor unclean persons of any description, can live out half their days. It would be easy to show, and prove also, how the end of these things, even with respect to the body, is death; but I forbear, and shall finish the subject with the words of the prophet: The show of their countenance doth witness against them, and they declare their sin as Sodom, they hide it not; wo unto their soul, for they have rewarded evil unto themselves.