Let me start off by saying that as a man I'm sorry you have to go through something like this. My wife and I last year was in a similar situation. We both sat down and talk about it and it was I that suggested being poly. Let me say that you are not weird in your feelings.
I believe that your husband is trying to confuse you and make you believe that you are the problem in the marriage. I'm not a counselor but it was the 6 months of counseling that led us to this junction. From that we found that we both had a part in the way our marriage was going. So your husband needs to man up and accept responsibility for his part.
Now concerning the having a baby and loosing weight. Please, please, please do not have a baby with this man. This will not help the situation it will only make it worse. Furthermore, their are a lot of guys out their that like full figured women. To me size 12 is not big at all, you are probably more attractive than he realize. I believe he said this to make you doubt your self and believe that no one will want you at that size. Well he is flat out wrong. You have to feel good about you, and if you are happy at a size 12 then stay a size 12. Erin what ever you do, do it because you want to not because he feel you should.
The bottom line as a man move to Florida and leave the idiot, you probably would be better off. Also, my wife and I are from Chicago, we both grew up on the south side of Chicago. So we both read this and we feel your pain, stay strong and know that its all about you.