It doesn't bother me to force her to internalize some of the costs she is inflicting on me.
Similarly, I don't mind taxing polluters in proportion to the pollution they emit.
(As of 2014, this argument applies to all Americans.)
I wonder how retirement costs compare to lifelong cost of being obese. According to stats cited by the CDC (http://www.cdc.gov/obesity/causes/economics.html) about 9.1% of medical costs can be attributed to treating conditions associated with being overweight or obese.
To make a larger point, the problems of others are your problems as well. If I never have kids, an education system isn't something I should be concerned with by this logic, yet I'd hate to live in a country without a public one.
With health insurance where costs are regulated (such as obamacare or our current employer provided health care), both people will pay $2000.
She's not inflicting those costs on you. You're offering to pay. That offer doesn't obligate her to make things cheap for you.
If you don't like what it costs for you to feel good about yourself, that's your problem.