I'm thinking of expecting my girlfriend to be fit
Why do you think she needs to be fit?
Yes, it's a well known fact that GPs in our society increase their likelihood to get cancer at a younger age.
Having the desire for your partner to make changes to their lifestyle and even lose weight is completely legitimate when the desire is based on a concern for their health and well-being. Talk to her about your health concerns and try to get on a program that works best for the both of you.
I'm still going to tell her she needs to lose weight. I don't care about her health.
Well you should care about her weight, and that should be the motivating factor behind why you want her to lose weight.