Ditto to everything Gala said.
I just finished reading a book that revolved around infertility and infidelity. The message I saw in it was how a woman ultimately caused a lot of pain and destruction all around her, the ripples spreading out to a number of people, because she wanted what she wanted and was determined to have it at all costs. (And there was nothing wrong with what she wanted; what was wrong was the absolute unswerving determination to have it at all costs, the attitude that she should and must have whatever she wants in life, regardless of the cost to herself or anyone around her.)
I will add, too, you are learning something about her. You were willing to come clean to your wife and live honestly. M is willing to lie to those closest to her. Are you going to be surprised when one day you find she's not honoring her agreements with you? Or that she's lying to you?