From Nagarjuna's Treatise on the Great Perfection of Wisdom
(Dharmamitra Translation)

Question: If the husband does not know, does not observe it and is not afflicted by it, what offense does the other party incur?

Response: It is on account of one's engaging in misconduct that it is then referred to as misconduct. This is a case of doing what is incorrect. Therefore there exists an offense. Moreover, there are all sorts of transgressions inherent in this. The feelings existing between husband and wife are such that although they are of different bodies they are the same in substance. If one steals the object of another person's love and destroys her original thoughts [of affection for him] one qualifies thereby as a thief. And so one commits yet another serious offense. One gains a bad name and ugly reputation. One is detested by others and experiences diminished happiness and increased fearfulness. One may live in fear of brutal punishment. Additionally, one is fearful that the husband and other people will find out about it. Hence one is much involved in maintaining lies. It is an activity which is denounced by the Aryas. It involves offenses within offenses. (The notes in red state: "Regarding this offense of lust, because during sexual misconduct one breaks [other] precepts it refers to 'offenses within offenses.'")

Furthermore, the sexually dissolute person ought to consider to himself, "My wife and his wife are both women. In terms of bone and flesh and demeanor, that one and this one are no different. And so why do I perversely bring forth these deluded thoughts and pursue such incorrect intentions?" A person who engages in sexual misconduct destroys and loses [any] happiness in both this life and later lives. (A fine name, a reputation for goodness, and peace and happiness of body and mind are gained in the present lifetime. The benefits of being reborn in the heavens, gaining the Way and reaching nirvana are realized in later lives.)

Then again, one should turn one's situation around and change places as a means of controlling one's mind, considering, "If he were to violate my wife I would be enraged. If I were to violate his, how would he be any different?" Having controlled oneself through such self-sympathy one ought then to be able to refrain from committing such acts.

What's more, as the Buddha said, a person who engages in sexual misconduct later falls into the hell of sword trees where manifold sufferings are experienced in abundance. When one succeeds in emerging and becoming a human, one's family life is not harmonious. One always meets up with a licentious wife who is devious and remote and ruthlessly cruel.