We know acting on a temptation can create karma. Does the fact of even being tempted to begin with suggest karmic issues that need deep healing?

This may very well be the case and is an insightful question for it speaks to human nature and how it comes to be and what those aspects of human nature truly represent. They are often something that is malleable, not carved in stone, but representing something that has been put in place, perhaps from prior lifetimes of experience, but not necessarily in the best way and for the best reasons, and may represent as much a liability as a source of inner strength and purpose especially if they give rise to inner conflict and fear. In the example you started with here of thoughts of infidelity, the obvious thing to consider in why this is happening is whether the person has a satisfactory partnership within the marriage or what is felt to be a permanent arrangement. There is a common misunderstanding in this regard with the many who propose open marriages, polyamory, and so on, as lifestyles. We can tell you that is unnatural. It is natural only in the sense one seeks to have intimate encounters with others of the opposite sex because that is an imperative of the gender makeup and the biology wanting an outlet in sexual activity. But beyond that, in terms of commitment to a stable partnership honoring vows of fidelity in particular, when temptation becomes strong, that is a clear indication that something has been missing in the marriage all along that has left a void, and it is likely what the person sees in their new flirtation is something they think will fill that void that has been present and seen as a deficit, and there is a yearning to fill that in. While you cannot change reality in certain respects—you cannot change how your partner looks or what they do for a living, or even persuade them to give up all the habits you may not like, it may be too difficult for them or impossible for them to change many things about themselves. This is why it is so important to look before you leap and consider the possibly very, very long future you must spend with a person in order to do justice to a lifelong partnership vow of fidelity. But within the reasonable bounds of acceptability in having various criteria in a mate, given a basic complement that meets minimal criteria, once a relationship is formed and agreed to, there are many ways to moderate feelings and find ways to focus on the positives and see that, in most cases, any apparent lack is as much the responsibility of the partner to take action in some way to alleviate the concern. In most cases, they will not be able to change their partner necessarily and so must adapt and find another way to govern their feelings through a new perspective to appreciate something that is present in abundance and focus more on that and realize that what they might be missing is not so very important after all. It might look enticing because there has been a lack for so long, but in the broad scheme of things it is still a small component of the attributes their partner does exhibit that will likely represent a core complement of benefits for the partnership to continue. Most things are workable and this is why many, many marriages are saved with some counseling to help people re-evaluate their goals, expectations, and get past some of the things that might be creating an estrangement that makes the superficial characteristics seem more important than they truly are. If people are not close and not sharing a true intimacy, they will be unsatisfied and there is no substitute for that. If this is pinned on the looks of the partner or some other characteristic or attribute that seems to be a dealbreaker or that last straw the person can no longer put up with, this may create a rash decision to separate when the problem is really more one of a lack of closeness. It is the sharing of intimacy people seek first and foremost and that goes a long way to fueling sexual desire in compatible couples but in its absence, everything can break down and people may end up disliking one another quite intensely and lead to a breakup. The willingness to be intimate and the denial of intimacy as a kind of payback through resentment, or intended as a willful punishment for a perceived slight, becomes very destructive, and here too when there has been an estrangement this leaves a vacuum and then people may see greener fields over the fence and want to stray, but it is the intimacy they crave more so than the accoutrements of someone new who looks good on the surface and becomes a source of temptation. Finding a way to heal the intimacy gap is the surest path to resuming closeness and acceptance of the other party and helping a marriage thrive without mistakes that can create an insurmountable breach of trust because they are felt to be unforgivable.