Aaron thought that it was Kayla who first brought up the idea of opening the marriage, because she thought it was what Nick wanted. I can believe that, but I think there's a chance she wanted it for herself too - the emotional affair with the Trekkie guy and then role playing Aaron Imholte being her soulmate? I don't even hate the guy, but let's face it - Aaron Imholte is no one's soul mate.
I think Kayla was so emotionally/romantically and perhaps sexually dissatisfied in her marriage to Nick that she was desperate for a grand love story. It didn't matter who it would be, as long as he would be willing to larp it with her. The fact that her and Aaron "fell in love" - in a house full of dirt, unwashed dishes, clothes, neglected kids, just shows how much Kayla is escaping into her own fantasy world. I don't think that's new. The way she's always been on Nick's streams, she's never been fully "there".
Oh, maybe someone will know. I remember there was also a story about Kayla joining the local community theatre and possibly getting close to some guy there too? But I don't know the source or how much of it is speculation. I'd like to know more, Kayla interests me.