Augustine said:
Passion is the evil in adultery. If a man has no opportunity of living with another man's wife, but if it is obvious for some reason that he would like to do so, and would do so if he could, he is no less guilty than if he was caught in the act. [i]

Country gentleman and a former U.S. president, Jimmy Carter said:
I've looked on many women with lust. I've committed adultery in my heart many times. God knows I will do this and forgives me. [ii]

Adultery is not just a sin to prudish Christians denying their natural instincts. Every people group through time has rules about cheating on your spouse. Inevitably, there are consequences to adultery.

In Native American cultures, a husband on an adulterous wife could impose severe penalties. In many instances she was made to endure a bodily mutilation that would, in the mind of the aggrieved husband, prevent her from ever being a temptation to other men again. [iii]

Chinese husbands who committed adultery against their wives were punished with castration, required by ancient laws. While men were castrated, women who committed adultery were punished by confinement. This punishment was the law during the Zhou dynasty. [iv]

In the Koran it says:
"Do not go near to adultery. Surely it is a shameful deed and evil, opening roads (to other evils)." Qur'an 17:32

Jesus turns the spotlight of God’s Love to a dark corner—private thoughts. If you think that you are righteous, look deep inside. Some people think, “What’s the big deal? What does it matter if you keep the thoughts to yourself or privately view pornography? Some believe that it’s not sexual sin unless you have intercourse outside of marriage. To the mistaken notion of ‘technically being a virgin’, Jesus tells the truth. If it happens in your heart, it is breaking God’s Law.

In the cultures surrounding Jesus, each had laws with harsh consequences for disobedience. And no matter the rule, there were many lawbreakers.

Adultery in traditional Judaism applies equally to both parties, but depends on the marital status of the woman (Lev. 20:10). Though the Torah prescribes the death penalty for adultery, the legal procedural requirements were very exacting and required the testimony of two witnesses of good character for conviction. The defendant also must have been warned immediately before performing the act. [v]

[iii] Schoolcraft, Historical and Statistical Information Respecting the History, Condition and Prospects of the Indian Tribes of the United States, I, 236; V, 683, 684, 686.
[iv] Royal Asiatic Society of Great Britain and Ireland. China Branch (1895). Journal of the China Branch of the Royal Asiatic Society for the year ..., Volumes 27-28. The Branch. p. 160. Retrieved 2011-01-11.
[v] Maimonides, Book of the Commandments, Prohibition 392 and the note at the end of Prohibition 347, Hebrew translation and notes by Rabbi Joseph Kapach, Massad Harav Kook, Jerusalem 1971