I've thought about this for a long time. I'm open-minded about it. But ideally, mutual gaining may be a better fit for me. I'd like to grow with my partner and get fatter, lazier, and sexier together. For me, I also think that having the same relaxed, mostly sedentary lifestyle as my partner would probably be more compatible.
That said, I also know I am more inclined to open relationships and having a hot fit friend to help me get bigger with regular encouragement is SO sexy. I want to have all the cakes and eat them all too I suppose.