There are many interesting biological differences that sciences may take note of & inquire after in a future classless society without racial & gender oppression (hence without race & gender). However, human biological differences do not have to resolve themselves into a two-sex, two-gender model that we have now. At present, I'm thinking that in a hoped-for communist future, we'll have an X-sex, zero-gender model. We don't live in such a society now, so I don't think we can quite imagine how many sexes (one, two, three, four, twelve, or more?) future communist citizens of the world may like to have as useful categories. For more, please see my December half Carrol's post on the question.
three cheers for older, politically mature humans,