Extreme obesity

because more is fat more is beautiful, sensual and sexy for many men here :-)

Having become immobilized by your fat cannot be a comfortable state of being. Sexually thrilling, yes, but physically hard to live with, I'm sure. Taking care of typical body functions becomes almost impossible - and having all that fat everywhere constantly rubbing up against neighboring fat, along with the inability to sleep on your side or even sit up, certainly makes for a difficult life.
And yet there are real people out there who will do whatever it takes, no matter the expense, the difficulty, or the danger, to appease their sexual appetite for their own enormity.

Even if she wanted such a thing, I wouldn't be party to it. Her life would be shortened for sure at such an enormous weight and I wouldn't want my participation in that tragic outcome to be on my conscience.

Wanting someone to become immobile to satisfy your sexual desire is a selfish fetish - but fetishes are, by definition, selfish interests.
As much as we love it, weight gain is, in truth, an unsafe practice for anyone to partake in. Yet it's such a turn on it would be hard for me to refuse a woman who was willing to do that for me.

I would.

I certainly understand the attraction.
Were it only not so unsafe.