None of your business op.
I'm sorry but that is a ridiculous thing to say. Of course it's OPs business - the essential element of an exclusive relationship is that you agree to have sex only with each other. This is what distinguishes it from a mere friendship. A sexual relationship involves finding the other person attractive. If she no longer finds her partner attractive because of her weight, of course it is her business!
Describing OP as "an abuser" is crackers! There is nothing here to suggest that at all. Being overweight is unhealthy; like it or not. Equally most people look better when not overweight; again like it or not, it's true. A lot of fat is basically unhealthy looking and unattractive.
I think your best option is to take up something together that will help - going to the gym regularly, a dance class, swimming, get a dog and long walks - it doesn't matter as long as you do it together and both enjoy it.
Ultimately, if there is no change and this is a deal breaker for you though you are going to have to broach it with her. Better that and give her the opportunity to lose weight rather than just dump her without explanation.
People round here go a bit crazy when it comes to weight issues (probably because so many people have their own weight issues and feel very vulnerable when weight and lack of attractiveness are discussed). But rationally, if a partner were doing something else that you didn't find attractive, many people wouldn't have an issue with dumping them.
"My partner has taken up smoking. I hate it and it's a turn off. I've asked her to stop."
"My partner drinks too much and is a nightmare when she's drunk. I've asked her to stop."
Weight is more of the same. It maybe a reflection of unhappiness and a deeper psychological problem; it may not be.
But people have their own dealbreakers.