The question contains a misleading formulation, highlighted in underline in the following excerpt:
It's obvious that the wife committed an injustice toward A because she broke a promise of fidelity.
One can also argue that B was being immoral toward himself by sleeping with a woman who is dishonest.
The question is was B immoral toward A?
The formulation is more precise if "immoral" is changed to "unjust." It is the virtue of justice that most directly applies to relations between people. Morality pertains almost entirely to one's own choices and actions in relation to one's own life and to the nature and demands of man's life qua man. Relations with others are certainly important, too, but mainly as an issue of justice, which is only one aspect of morality.
If the issue is specifically injustice, then yes, the wife and the guest are both being unjust toward the wife's husband, blatantly so under the circumstances described in the question, i.e., two brothers on good terms, with one brother seeking in good faith to help the other in a time of temporary need. That is not a rationally appropriate way for the needy brother to act; it is an immoral course of action, contrary to the requirements of man's life (including his own life) under the stated premises of the question.
answered
Aug 24 '12 at 01:53
Ideas for Life ♦
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Would B violate the trust of A by covertly enjoying his wife's sexual favors? That's unjust. Would B fake facts to maintain that trust while accomplishing his covert actions? That's dishonest. Would B be using dishonesty to enjoy something of A's which would not be willingly granted (i.e., the sexual favors of his wife, rather than the sexual favors of a mere ex-wife which he has no say in)? That's fraud. On and on, in most contexts B would be deeply immoral to do such a thing, and yes it would mean being dishonest and unjust to his brother.
What does it mean "not willing to grant sexual favors of his wife"? Are you implying some sort of ownership over services by the recipient?
Absolutely not. If someone doesn't want to share the sexual favors of their wife, they can stop being her husband.
Does A have some sort of exclusive rights (not ownership) to wife's sexual services that B violated?
Please notice that I framed the above in terms of hypotheticals. Any exclusivity would of course be an agreement between husband and wife.
Let assume that there is that exclusivity agreement between husband and wife. Is it solely the responsibility of the husband and wife to maintain the integrity of that agreement or is the brother also somehow responsible?
"Act immorally toward" is an awkward and unnecessary construction. There is injustice, which is toward a person, and there is acting immorally, which has no specific object. Acting immorally is like acting stupid. Actually, they are virtually identical.
Immorality and stupidity are completely different. A stupid person can still act perfectly morally. Any person with free will can act perfectly morally.