This may be one of those situations where she feels as though she has fallen out of love with you, and her "temptations" to enjoy other men flirting with her is redefining her own childhood traumas.
If she went to therapy and it only made it "worse" is because she is unable to accept that she and her mom may be just alike.
If your wife is unable to accept who she is and love herself (desires for other men and all), then she will have to reject those ideas and you lie in bed beside her at night while you have to reject the ideas of being with her. If this is the case it is important to realize that the two of you are experiencing the same amount of pain. Try and get her to talk about it.
I totally agree with this ^
Usually childhood trauma will manifest itself in later years when one finds one going down the path that traumatized them. Otherwise, there is nothing there to spark those 'flashbacks'.
Your wife's mother had multiple sexual partners outside of marriage, and did not hide it from her daughter. Given the stigma one gets (especially women) from having multiple sex partners, it does not go a long way to teaching one's children how to have long-lasting, serious relationships.
But OP's wife has, and does have a long-lasting relationship, despite this. Sex was infrequent (likely due to the negative connotations sex had, thanks to her mother), but nonetheless, there was a sex life - one that OP was at least okay with.
Now, all of a sudden, it's manifested itself in his wife. Why? Likely because of exactly what badsanta said above - she's starting to have thoughts of desire for other men (or one other man). Maybe even a celebrity. Maybe a book that has made her think of having sex with somebody else, fictional or otherwise.
In other words, it's occurred her that perhaps she's just like her mother (even though that's likely not true).
All of these years spent with one person, and she's probably only ever had eyes for him, thoughts for him, interest in him. She's consciously or subconsciously become the antithesis of her mother - hooray!
Then OMG, she realized she was able to be aroused by somebody/something else. Has nothing to do with her husband - all of us can be aroused by someone/something other than our spouses, but we don't usually have this kind of shame associated with it.
It's as if her mother was a raging alcoholic, and a bad drunk. She grew up believing that alcohol was evil, and she'd never fall prey to it the way her mother did. So she only ever drank socially, and never to excess. She resisted that temptation to ever drink more than one glass of wine every couple of weeks or so, until she realized she liked wine. Uh oh.