It sounds to me like she is looking for a challenge and to be pursued. She HAS you and not this other individual who is paying attention to her wooing her and this other individual is probably getting the same ego trip from her even though he is married as well. She wants to feel "desired" and often in a marriage that can fade and a couple can get all about servicing rather than keeping the other partner's ego pacified.

When was the last time you really wooed her? Took her out, told her how hot she is and that you just want to climb on her and devour her?