When is Infidelity Acceptable? By Miss Know it All

Miss Know It All
I read an article in the November 2011 issue of AARP Magazine titled, When the Vow Breaks written by Mary A. Fischer. The piece begins with a husband whose wife is suffering from dementia, and how he spends every Saturday afternoon with his girlfriend.

After reading this editorial, the question is, "Is infidelity ever acceptable if a significant other is physically or mentally incapacitated?"
When couples vow to remain faithful, for better or for worse and in sickness and in health, sometimes that promise can become ambiguous if a spouse is not able to give or receive emotional or physical fulfillment because of a debilitating illness.
Most people agree that there is never any excuse for stepping outside of a marriage. However, some health professionals, social workers and religious leaders are beginning to redefine what adultery is when failing health becomes an issue. A few experts believe that establishing a relationship outside of the marriage could be beneficial to the caregiver and the person, who is suffering from a deteriorating disease.
Adultery is defined as sexual unfaithfulness of a married person. If a new definition of infidelity is suggested, perhaps the declarations recited during a wedding ceremony may also have to be modified.
What are your thoughts about this issue? Is it ever okay to commit adultery if a spouse cannot fulfill his or her partner's physical or emotional needs because of fading health? Do you know of any couples who are presently going through this dilemma?