For out of the heart proceed evil thoughts, murders, adulteries. Matthew 15:19

For the woman which hath an husband is bound by the law to her husband so long as he liveth; but if the husband be dead, she is loosed from the law of her husband. So then if, while her husband liveth, she be married to another man, she shall be called an adulteress: but if her husband be dead, she is free from that law; so that she is no adulteress, though she be married to another man. Romans 7:2-3

Know ye not that the unrighteous shall not inherit the kingdom of God? Be not deceived: neither fornicators, nor idolaters, nor adulterers, nor effeminate, nor abusers of themselves with mankind…shall inherit the kingdom of God. 1 Corinthians 6:9

My child, be not a lustful one, for lust leads to fornication. Be neither a filthy talker, nor of lofty eye, for out of all these adulteries are engendered. Didache (A.D. 80-140) ch. 3

"Sir," say I, "if a man who has a wife that is faithful in the Lord detect her in adultery, does the husband sin in living with her?"
"So long as he is ignorant," said he, "he sins not; but if the husband know of her sin, and the wife repent not, but continue in her fornication, and her husband live with her, he makes himself responsible for her sin and an accomplice in her adultery."
"What then, Sir," say I, "shall the husband do, if the wife continue in this case?" "Let him divorce her," said he, "and let the husband abide alone: but if after divorcing his wife he shall marry another, he likewise commits adultery."
"If then, Sir," say I, "after the wife is divorced, she repent and desire to return to her own husband, shall she not be received?"
"Certainly," said he, "if the husband receives her not, he sins and brings great sin upon himself; nay, one who has sinned and repented must be received, yet not often; for there is but one repentance for the servants of God. For the sake of her repentance therefore the husband ought not to marry. This is the manner of acting commanded on husband and wife.
For this cause you were commanded to remain single, whether husband or wife; for in such cases repentance is possible."
"I am not giving an excuse that this matter should be concluded thus, but to the end that the sinner should sin no more. But as concerning his former sin, there is One Who is able to give healing; it is He Who has authority over all things." Hermas (A.D. 150) Ante-Nicene Fathers vol. 2 pg. 29

Concerning chastity, He uttered such sentiments as these: "Whosoever looks upon a woman to lust after her, has committed adultery with her already in his heart before God." And, "If your right eye offend you, cut it out; for it is better for you to enter into the kingdom of heaven with one eye, than, having two eyes, to be cast into everlasting fire." And, "Whosoever shall many her that is divorced from another husband, commits adultery." And, "There are some who have been made eunuchs of men, and some who were born eunuchs, and some who have made themselves eunuchs for the kingdom of heaven's sake; but all cannot receive this saying." So that all who, by human law, are twice married, are in the eye of our Master sinners, and those who look upon a woman to lust after her. For not only he who in act commits adultery is rejected by Him, but also he who desires to commit adultery: since not only our works, but also our thoughts, are open before God. Justin Martyr (A.D. 160) Ante-Nicene Fathers vol.1 pg.167

But to put to shame and to discourage those inclined to contract a second marriage the apostle appropriately uses strong language and says at once: "Every other sin is external to the body, but he who commits fornication sins against his own body."… as idolatry is an abandonment of the one God to embrace many gods, so fornication is apostasy from single marriage to several. Clement of Alexandria (A.D. 195) Miscellanies, book III ch. 12