OP, assuming it was not a silly joke (and I don't think you'd be posting if it had been) I would be scared to be a man like that. First, you are slim. Not even close to the upper end of the healthy weight range, let alone overweight. 7 stone is underweight and unhealthy. I'm 5'4 or 5'5 depending on whether I stand up straight, and as an adult I've never weighed below 8 stone, have often been 9 and something and everyone has said that is slim and looks good. Frankly, if he thinks of anything in the healthy weight range up to about 10 and a half as fat, then he has a problem. Do you think he could be gay, or into very young girls? I just think the average heterosexual man, or man without some kind of outlandish sexual preference would see you as ideal right now. To see you as fat is a problem he should seek help with.
To think that your weight, whatever it was, is something he should forgive you for would be wrong even if you were overweight, the fact you're not adds a sinister twist. What exactly does he think a wife is there for? Not be loved for who she is, obviously.
I think you should say to him - 'Well, it is worrying and very strange that you would say that, because I'm not fat. If you don't find me attractive at this size perhaps you should see someone about your obvious problems'. Then add that 'having your child may involve a temporary change in weight, or a change in figure shape, as might any number of things in the future (illness etc). So actually, given that you are a very superficial person I don't feel you'd make a suitable partner or second time father at this point.' Ask him to reflect on his outrageousness and come back to you with an apology. Then leave the room. If he doesn't come up with something excellent to explain himself, you might want to take note and mull over what you want to do.
I have been many different weights while I have been with my DP, so has he actually. Neither of us has ever said that it has stopped the other being attractive (it hasn't, for me anyway and he seemed interested 
). Nor have we ever said that someone should be forgiven, or feel that love and affection from their partner depends on their weight. I know we both look and feel healthier at an ideal weight so we try to lose weight when we do gain it. In friend's and relative's relationships, again all kinds of weight changes over the years and support given.
So your DP's attitude is very abnormal and mean, even if your were hugely overweight.....how bizarre and concerning is it then, that he has this attitude when your weight is pretty much ideal?