Unfaithfulness to the marriage vow can never be justified.

The forsaking of all others is a keeping of faith, not just with the chosen one, but with the ones forsaken. The marriage vow unites not just a woman and a man with each other; it unites each of them with the community in a vow of sexual responsibility toward all others. The whole community is married, realizes its essential unity, in each of its marriages...
Marital fidelity, that is, involves the public or institutional as well as the private aspect of marriage. One is married to marriage as well as to one's spouse. But one is married also to something vital of one's own that does not exist before the marriage: one's given word. It now seems to me that the modern misunderstanding of marriage involves a gross misunderstanding and underestimation of the seriousness of giving one's word, and of the dangers of breaking it once it is given. Adultery and divorce now must be looked upon as instances of that disease of word-breaking, which our age justifies as "realistic" or "practical" or "necessary," but which is tattering the invariably single fabric of speech and trust.
(pg.117, "The Body and the Earth")

Sociologists argue that in contemporary Western society the marketplace has become so dominant that the consumer model increasingly characterizes most relationships that historically were covenantal, including marriage. Today we stay connected to people only as long as they are meeting our particular needs at an acceptable cost to us. When we cease to make a profit - that is, when the relationship appears to require more love and affirmation from us than we are getting back - then we "cut our loses" and drop the relationship. This has also been called "commodification," a process by which social relationships are reduced to economic exchange relationships, and so the very idea of "covenant" is disappearing in our culture. Covenant is therefore a concept increasingly foreign to us, and yet the Bible says it is the essence of marriage.

You have my whole heart. You always did.

In sharp contrast with our culture, the Bible teaches that the essence of marriage is a sacrificial commitment to the good of the other. That means that love is more fundamentally action than emotion. But in talking this way, there is a danger of falling into the opposite error that characterized many ancient and traditional societies. It is possible to see marriage as merely a social transaction, a way of doing your duty to family, tribe and society. Traditional societies made the family the ultimate value in life, and so marriage was a mere transaction that helped your family's interest. By contrast, contemporary Western societies make the individual's happiness the ultimate value, and so marriage becomes primarily an experience of romantic fulfillment. But the Bible sees GOD as the supreme good - not the individual or the family - and that gives us a view of marriage that intimately unites feelings AND duty, passion AND promise. That is because at the heart of the Biblical idea of marriage is the covenant.

...We must say to ourselves something like this: 'Well, when Jesus looked down from the cross, he didn't think "I am giving myself to you because you are so attractive to me." No, he was in agony, and he looked down at us - denying him, abandoning him, and betraying him - and in the greatest act of love in history, he STAYED. He said, "Father, forgive them, they don't know what they are doing." He loved us, not because we were lovely to him, but to make us lovely. That is why I am going to love my spouse.' Speak to your heart like that, and then fulfill the promises you made on your wedding day.