this issue is just one of many you have to settle before marriage: what physcial, emotional, spiritual, psychological health means to you, what you are willing and able to do individually and as a family to insure the health of the parents and children, and how you will cooperate as a couple in providing this for your family.
I don’t think the guy is worried that she might look a little porky when viewed in slacks from the rear, he sounds like he is worried she already has a chronic health issue, which will only get worse, if she does not take care of herself.
Having been on both sides of the battle, I can only state my opinion that a spouse who abuses themself, either through poor eating habits, lack of commitment to whatever medical program is prescribed, lack of exercise, is just as guilty of damaging the relationship as a spouse who uses illegal drugs, drinks or otherwises engages in self-abuse.
A married couple is literally one flesh, and to abuse oneself is to abuse one’s spouse, and this is a sacrilege against the sacrament of marriage. In fact, some psychologists would say this person is engaging in passive aggression in so doing. I am aware this is a very strong opinion but through observation and experience, and extensive counselling dealing with related issues, I think it is pretty sound.
bottom line, resolve this before marriage, the underlying attitude not just the behavior or outward appearance, or it will come back to haunt you. also remember the little fable about the pot and the kettle-- FW is right about leading by example.