Man; not sure that M has decided quite what she'd be interested in yet. A relationship of some? any? kind? Maybe not. She seems a bit impulsive and immature at this stage in her life; some people have to grow out of the phase when they're all about getting attention, stimulation, and drama, you know?
So I'm getting that you and W aren't exactly thinking about "doing" poly in the future; you're just trying to make sense of how this could have happened right now. People have feelings for other people. It happens. All the affairs and divorces you read of in the tabloids will tell you that. Polyamory just offers a potentially positive way to deal with those wandering emotions.
You'll find, especially upon delving into those two books I've mentioned, that "wandering eyes" isn't necessarily at all a sign of something being wrong in the original relationship. Honestly, it really is just human nature. At the same time, I maintain that some folks are best off avoiding poly like the plague. And yet, it doesn't hurt to know what poly is and how it works.
Please keep us updated here on Polyamory.com on any little way we might be able to help.
Love means never having to say, "Put down that meat cleaver!"