I have been very upset about this news. Its devastating to know she was lying to me the entire time about this. I feel so betrayed.
Of course they were lying. ALL CHEATERS LIE. Period.
There's almost always another person. And they will ALWAYS lie about it, or - at best - cop to one level LESS than one's really going on.
I'm sorry you had to hear about this, but -- like you said -- at least you now have some explanation for her behavior. Affairs really do change them physiologically.