So would you be OK with an employer asking if you have a spouse and children, since putting dependants on employer health insurnace benefits will cost the employer more?

It would be okay with me. If a single person provides an employer a cost/benefit advantage, then that person should have a hiring advantage. Happens all the time in many fields. A single person who can travel, work overtime, work holidays has an advantage over someone who for family reasons is more restricted. Not sure why health costs should be left out of the equation.

The more complex answer is that to my knowledge, employees with families typically pay a higher health premium than single employees. The added cost is shifted to the employee.

Why should the non-smoker/obese person get to "appear" lower cost, when they could have a chronic condition that's just not visible, or they could have dependants with chronic conditions?

Imagine yourself an employer of a small business barely surviving the economy. You have two candidates for a job opening who are equally qualified. One is normal weight. The other is obese. Your accountant tells you that over the next 10 years the obese person is likely going to cost you an extra $50,000 in higher health premiums and lost work days due to health reasons. Who would you hire?

Or suppose you are an employee for a small company. You find out that if the company hires a couple of obese people, it would raise health premiums for all employees $200/year. Would you want your company to have a BMI policy?

Any analysis is subjective to what you believe the employee will cost. You could assume female employee costs more because on average women tend to go through pregnancies - which would mean you're fine for an employer legally discriminating against all women, even though any particular woman might not want to have children.

Pregnancies represent a special case. We as a society agree that being able to have and raise children are good things and so we pass laws to facilitate those to occur. This includes laws against sex discrimination for the reasons you outlined.

In contrast, we as a society do not agree that obesity is a good thing.

Keep in mind that I'm not arbitrarily picking on the obese because I hate fat people. The facts are that health care costs are a big problem and that the rate of health cost increases will soon make it unaffordable regardless of the payment system we finally adopt.

The single biggest cost factor in health care are complications caused by obesity. The second biggest is smoking.