What is with all this, "find someone who will accept you as you are" crap? I'll grant this is an FML because that's just an awkward moment to bring it up, but perhaps OP's fiancé doesn't want to watch them die younger than they should because of diabetes or heart disease. If OP is big enough to bring about legitimate health concerns then OP's fiancé should most certainly not just accept it, any more than they should "just accept" if OP was an alcoholic.

To some degree, I think that it is not just a matter of this guy being an asshole. As #102 said, there is a risk of health problems one faces due to the nature of weight issues. I myself have diabetes because I am overweight. My own grandfather died due to complications of heart disease that is linked to his weight, and to diabetes. The problem I have is the method of how your boyfriend delivered this request to you. "Get thin first" is an inappropriate way to make the request that someone he claims to love lose weight. It also makes me question the motive behind his request. Is it just shallow that he wants you to look good, or is he genuinely concerned about your health? If it is the latter, then ask him to be a little more considerate in how he makes the request. "Get healthier" might have been more appropriate, and certainly not part of the proposal. It is just a tacky way to make any request regarding such things.

Yeah, I know this situation. Although the guy has never seen me thin and I have always been that fat. Don't know why he even bothered to propose.

Date someone who loves you for who you are, not this douche bag...

Dump his ass.

You...you said no, right? Proposals should not be conditional. That's like saying to a balding man, "Sure, I'll marry you if you grow your hair back." Metabolisms change, and they're different to begin with, too. For anyone concern-trolling about her supposed health, first, you don't know her size or what her boyfriend judges as thin, and second, "thin" isn't a measure of health. So he's being a shallow jerk whose love is dependent on physicality. Gods forbid she ages, or becomes pregnant or ill, or anything else that may impact her looks.

Doesn't deserve you.

Leave him. Lose weight anyways and get yourself looking sexy, and find another man.

Maybe OP's weight has been impacting job performance, health, or the relationship itself. Marriage would be a great motivator, IMO. We lack context, making it hard to hear the proposal in such a sour tone.

Keep looking for someone who does.