A lot of mine run along the lines of: I'm in a relationship with a BBW, who happily gets fatter and more out of shape for a while, because it turns her on as much as it does me. Until she feels that she's gone farther than she is happy with...but she is still turned on by the whole concept, so while she wants to lose a little weight and get into a little better shape, she wants me to gain and get out of shape. I'm good at gaining weight and being sedentary, so quite quickly I'm as heavy as she is, at which point she starts really taking charge of most activities and pushing me to be even more sedentary. When we realize that I can't keep up with her, physical fitness-wise, despite her still being very fat and out of shape, it is a huge turn-on for both of us. I realize that, like she had earlier, I've really gotten a bit too fat and out of shape to deal with life easily, but we decide that I'm not going to turn back, and I continue to become massively flabby, huffing and puffing just to walk a few dozen yards.
14 years