Of course it is cheating whether it be rnt or fs. Think if it was on the front page of a new. "Punter found cheating at brothel/massage parlour by wife." if you get caught it is how you get deal with it afterwards.
I guarantee most wives know.
I've seen a few unhappy and unsatisfied wives for secx and hear all about hubby's extra curricular activities and the excuses that are used.