First, I didn't mention nebulous, untested theories about mental health or social science. I mentioned testable, verifiable, commonly known, provable conditions such as the numbers of X and Y chromosomes. In other words, I was saying that gravity exists, not that a specific controversial theory of gravity is true.
Secondly, even if science is not settled (a true statement), and can always be improved upon (also true), there is this: In science, a "fact" is something that is so confirmed that it is perverse to withhold provisional assent. The fact that having a single X chromosome and a single Y chromosome makes you a man is, indeed, a scientific fact. I don't think your comment justifies taking the opposite position...that because the science might change some day, we should base our laws on the possibility that science could be wrong.