In fact, the policy would not be grounded in reality. Some babies are not clearly male or female. Though parents might choose to define their children as one sex or another, a better term for many of these people is intersex. Others in this situation might find that the gender their parents chose for them does not fit and wish to switch. Genetic testing is not a reliable way to resolve questions about biological sex; international sports authorities have struggled to find a scientifically sound test, based on genetics or hormone levels, that reliably separates the sexes.
Fine, one might argue, but many transgender people appear clearly one sex or another at birth, so they are easier to define as male or female in an objective fashion. A brief conversation with a transgender person would cure most Americans of the notion that their expressed gender identity is a shallow preference, a phase or something other than a deeply held knowledge that their body does not match who they are. Arguments otherwise rely on the same warped logic that allowed so many people to write off gay, lesbian and bisexual attractions as a choice. The government can deny that some people who were born women know they should be men, just as it denied for so long that some men love other men or that African Americans and white people can build stable marriages and families. All this denial of reality would accomplish is once again to marginalize people.
It also would represent one more broken promise from President Trump, who vowed in his 2016 campaign to be more enlightened on LGBTQ issues than that.