Newsweek has an article about a possible "genderless" future, in which "no gender" would be one of the choices.
I am OK with legally acknowledging that some rare individuals do not have a clear sex or gender. And I think it is only civil to accommodate such people in social life in nearly every situation. The condition is so uncommon that I do not think we should build an entire third-gender infrastructure.
I do think it is a mistake to claim that gender is "assigned" for most people. For the vast majority of people, gender and sex are the same. We do not have to mentally disconnect the two in all cases just because they are not clearly connected in a few rare cases.