Very scary!!!
I would feel like it was cheating if the person he was fantasizing about was a real person in his life not a picture in a book or movie star etc but someone he is in contact with. Then it is only too real because his thoughts have the possibility of being actions!
However, if you believe that your man would never go down that track (there are men out there that are like that) then maybe it's just that your bedroom actions need some attention. After years of commitment things generally tone down in the bedroom and we forget about others needs...
So it does depend on the relationship between the two involved.
