i've never truly wanted to be immobile, but the fantasy is a very arousing one for me, under the right conditions.
early this year emily and i were on a weekend away, and i got a tiny glimpse of immobility... we had a romantic dinner out and we back at the place we'd rented. we shed our clothes and started to make out, and she said she wanted to feed me. i happened to sit on a reclining chair (the lazy boy style), and when she climbed up on me, the chair felt back into recline mode.
she fed me and teased me, and eventually when she was out of food she climbed off me. she summoned me to join her in bed. i tried to push the seat forward, but i couldn't get it to move. i was wedged in pretty tight between the arms, too so i couldn't shift my butt very much either, especially with the weight of my stuffed belly resting on my lap. i asked her for help, for her to help push the chair back into a seated position so i could squeeze out of it. as soon as she realized the position i was in, her eyes lit up.
that was the most realistic immobility play we ever had - i was really trapped by my own bulk. and it was really, really hot, as she teased me and had her way with me, and even forced me to eat some more!
so this has popped up in my fantasies from time to time since.