Shit happens in relationships. It's completely unrealistic to expect people to stay the same weight. Especially for women who want children. (Not saying OP wants kids.) What if she gets PPD and can't lose weight immediately? Is her boyfriend going to dump her because he has no grasp of biology.

Secondly: it takes YEARS AND YEARS of unhealthy eating to get to a point of losing a leg. And that's IF you develop type 2 diabetes in the first place. Just because someone is overweight doesn't mean they have diabetes. Or blood clots, or whatever else could possibly make them lose a limb.

OP's boyfriend is 1.) incredibly stupid; 2.) not worth being in a relationship with.