It would seem that you are right if we are to speak in good faith and with respect to general physical health. You would find very few people that disagree with the statement, “A five-hundred pound person is unhealthy.” However, such a person could view themselves as healthy and say, “But, I feel healthy.”
That’s beside the point. When you want to talk about an objective morality, then you’re going to be talking about an objectively correct moral decision, with exactitude. It would be like me asking you to point out a person who is in perfect physical health. You point at guy x, I say that guy x is a couple pounds underweight. You point at guy y, well, it turns out he has the beginning stages of liver failure and doesn’t know it.
When you discuss morality, you are not actually dicussing actions, you are discussing what you think of those actions. When you discuss physical health you are not discussing people, you are discussing what you think of those people.
Math is subjective because it is purely a human construct and is used predominantly to describe relationships. Math does not occur in nature, the relationships that Math describes occasionally do, but not Math itself. It’s basically the same thing with science, the basic goal of science is to try to determine what objectively exists, in terms of things and relationships, and to determine what is objectively true. We apply names to these things in the language of science. The chemical formula for Magnesium Hydroxide Mg(OH)2 is a human construct and is not a priori knowledge, that there is actual, physical Magnesium Hydroxide is, however, objective.