Can a one night stand ever be "OK " in a long term marriage or is it the same as an affair?

I would like to know your opinions on this - is a one night stand "justified" as a way to relieve the boredom of a long term monogamous relationship for a moment or is it the same as having a longer term affair when one falls for anther person and becomes attracted to them and they occupy your thoughts all the time. This is not a good situation if you dont have the guts to break your long term relationship because one wonders is it better to stay with it and not plung oneself into turmoil of splitting emotionally and physically with another person you have been married to ( 20 years). The future can be scary if one is not emotionally mature to cope with all the changes that will come. On the other hand I am not getting any yoounger and was a virgin when I married and would like to experience with another woman - I feel somehow incomplete - is this all just rampant narcissism and selfish - dont know what you have till its gone thing - life's choices are hard?