There are gender-specific laws in the US. For example, selective service registration, which is mandatory for males. Without having a legal definition there is no way to enforce this law since everybody will be able to claim not being a male to evade it.

> There are gender-specific laws in the US.

Losing the distinctions in those laws would mostly be a net win for society.

For those that address a real need with sex distinction, if any, you can just define things in terms relevant to the law rather than a general purpose “legal gender” and have better laws without externally imposing broad gender labels.

I don't think drafting women, for example, is going to be a net win for society. But nobody prevents you from campaigning to repeal or amend those laws to remove gender. But while we still have such laws there has to be a legal definition of gender, it seems pretty obvious.

> I don't think drafting women, for example, is going to be a net win for society.

If having a draft is good for society, especially a limited, skills-based draft (one of the things that the selective service system in principle supports), then excluding women from registration for it an exposure is a net loss (even if you might also benefit from excluding or limiting exposure of people who might bear children in the future to the type of mass draft used for, e.g., the world wars.)

> But while we still have such laws there has to be a legal definition of gender

The question was “what is the harm of making legal gender optional?” Obsoleting distinctions in law that are not themselves beneficial is not a harm.

I am not sure this is how arguments work. Saying "if <something> then <something else>" works only if <something else> actually follows from <something>.