A: Adultery is not treated very extensively in The Urantia Book. It is mentioned only a handful of times, and then, mainly only in the context of evolution, where it had to to with ownership of women and property. There are a few mentions in the later portions of the book, and then, there is this event from Jesus' life:
In another event, Jesus mentions adultery in this vignette involving a young man who wanted assurance of eternal salvation:
Jesus was confronted by an angry crowd who had a woman in custody for being an adulteress. They wanted Jesus to approve stoning her to death, which was the custom of the day, but it was a situation where the woman had been ill-treated by the husband, and coerced into selling her body for money for the husband. Jesus knew, of course, the whole story, and through his superb discernment, de-fused the entire situation, and the woman was set free. Jesus had no words of condemnation for this woman. (162:3.1)
Since there is such a scarcity of real advice on this subject in The Urantia Book, I am going to try to answer your question, but from my own perspective, tempered by my years of study with The Urantia Book, and with life...
...as Jesus was going for a walk, [a] young man accosted him and said: "Master, I would know from you the assurances of eternal life. Seeing that I have observed all the commandments from my youth, I would like to know what more I must do to gain eternal life? In answer to this question Jesus said: "If you keep all the commandments--do not commit adultery, do not kill, do not steal, do not bear false witness, do not defraud, honor your parents--you do well, but salvation is the reward of faith, not merely of works." (163:2.4)
It is never a mistake to love another person—but we must make the distinction here between love and sex. They are not necessarily connected, nor should they be, if in doing so, you cause harm to another. That is what adultery is—not just LOVING a married man, but allowing that love to cross the line into a sexual connection. And such an act CAN cause harm, and almost always DOES cause harm.
Fortunately, adultery is no longer treated as a capital crime, warranting death. But, I believe that any thinking and feeling person can discern that there is something not right about it—in principle, if not in fact. There may be factors which contribute to a person's thinking that they are justified in having sex with another person's spouse. But in the final analysis, it is not a physically healthy, or morally correct thing to do. We can certainly love whomever we please, but that love should not cross the line into adultery.
Aside from the health aspect, there are the other spouse's feelings to consider. In its simplest form, adultery appears to me to be motivated by selfishness. And since true love is not selfish, it would also seem that adultery is neither loving nor uplifting for either party—certainly not for the betrayed spouse. And this does not even address the issue of children who may be involved, should this behavior jeopardize their relationship to either of their parents.
Here are a few Urantia Book quotes regarding love:
It appears to me that a person who is thinking about a liaison with a married man is thinking less about real, heartfelt, and honest love, and more about a selfish need for this person's affection, or attention. It bespeaks an ulterior motive that may not be healthy. And that is not love, but something else entirely, and not entirely good. Again, this is my opinion, but I would think that since the taboo against adultery is one of the original ten commandments, one might be wise to avoid it."
Love is the outworking of the divine and inner urge of life. It is founded on understanding, nurtured by unselfish service, and perfected in wisdom.
Love is the secret of beneficial association between personalities.
Love is the desire to do good to others.