SEX

"What then is unclean? Sin, malice, covetousness, wickedness. As it is written: 'Wash yourself, make yourself clean, put away the evil of your doings' (Isa. 1:16). 'Create in me a clean heart, O God' (Ps. 15:10) ...The man who had two wives was not unclean, and David who had many wives was not unclean. But when he had one unlawfully he became unclean. Why? Because he had injured and defrauded his neighbor. The fornicator is not unclean on account of the intercourse, but on account of the manner of it, because it injures the woman, and they injure one another, making the woman common, and subverting the laws of nature. For she ought to be the wife of one man ...Here then is injustice, and therefore the act is wicked."
--St. John Chrysostom, Homily III and IV on Titus I and II, in Johanna Manley, The Bible and the Holy Fathers for Orthodox, p. 589.