Cheating by definition is the act of doing something behind another's back,it doesn't matter if it's a man,woman or a fucking horse it's still cheating if she did it without your knowledge. If she told you beforehand and you agreed then that becomes your problem.
But it's a very cucklike behaviour which will lead to serious consequences when she starts asking for that more and more, what's to say she doesn't eventually work her way up to fuck a guy next time,maybe a collegue,your boss etc. Once you open that Pandora's box you need to know that you can't close that shit again.