Oh dear, I have been there. My ex h told me about 5 years ago 'I will not have sex with you again unless you do something to make yourself more attractive.'!!!!!!!
I am 5'5" have had three children and at the time weighed about 9 1/2 stone and although am no Catherine Zeta Jones, I'm not bad!
It really is a horrid thing to hear.
It can really break your confidence when the person whom you love comes out with hurtful comments like that, and confidence is a very fragile thing. Mine was shattered and it has taken ages to get back. Even now when I am with dp who thinks I am gorgeous (and I'm a lot heavier now!) fancies me to bits (and we have great sex and lots of it ) my confidence isn't what it was.
If you lose the weight and then something happens to you, lets say a car accident that leaves you disfigured, what would your dp do if he is so obsessed with your appearance?
Comments like this are a way of control, if you lose the weight, I suspect, although I don't know, that there would be another way of excersising control over you. I know that there was with me. I am not suggesting that you split up, leave him or do anything drastic like that, but I think that a few sessions at Relate may help you and him find out what the real issues are behind comments like this.
Good luck and loads of hugs