Her children will feel betrayed if/when she leaves their father to find fulfillment. The stable monogamous foundation she thinks is so important for them will be revealed as a lie, and their mother as a thrill-seeker.
Of course, her husband may leave first, if he isn’t getting any at home, and he finds a woman who appreciates him better.
I made it plain to my wife before we married that I had no intention of cheating (and I haven’t). I also told her that I wasn’t going to be celibate either. This doesn’t mean I get sex every time I want it, but if I was this guy, I would have been gone long ago.