I think that as long as it breaks an agreement of what both spouses agree is acceptable behavior for both parties it is a problem. Whether it's cheating is another question. This then poses the question about whether a man's wandering eye is cheating, or a woman's thoughts about being with a celebrity is cheating, or if sexual dreams are cheating.
On some level, man is a manifestation of the essence of man and woman is a manifestation of the essence of woman. We men appreciate the female, but commit to one that represents all of the female being, and vice versa. Just what that commitment entails is a combination of social conventions and the specific agreement between the two people in the relationship. As long as the actions bring justice to and honor the relationship, even if it is counter to societal norms, it isn't cheating. Only once a member of the union is hurt or feels dishonored, a boundary line either must be re-evaluated or a clearly defined boundary has been crossed and a wrong done. The trouble here is that societal norms play a big part in our internal value structure, but they don't dictate them. This is much the same as a child's value structure is built up around the parent's but in doing so is a response to key componants of it, and the subjective judgements may be polar opposites of those of the parent.
If you are hurt, communicate your hurt. If your spouse doesn't want to address this hurt, there may be deeper issues at play. Read the section in DR about this. Some suggestions are to get involved in the fantasy yourself - thereby fostering bonding and improving your relationship or give ultimatums if this behavior is totally unacceptable to you. There should be a way to come to a compromise, as this behavior is likely an escape/habit that can easily be replaced if both partners are willing to get creative.
“No problem can be solved from the same level of consciousness that created it. ” – Albert Einstein
