Esperanto has chosen all words to be masculine positive by default, they could have introduced a feminine-only word like French does though, such as mino.
Esperanto didn't choose anything, it being a language not a person. Zamenhof made a choice, but even he considered the alternatives back in the 1880s, albeit that he decided in the end that a balanced gender mechanism that had the masculine as a marked form was too foreign a concept to speakers of existing languages with unmarked and marked masculines, such as English.
In the more than a century and a quarter since, there have been efforts to fix this, as I said. Several of them are over four decades old at this point.
Use of the prefix "ge-" in the singular (giving "patro" for father, "patrino" for mother, and "gepatro" for parent of unspecified gender) goes back to the 1980s. This has in the decades since even that become mainstream enough that in the last 10 to 15 years "gepatro" is now in Esperanto dictionaries such as Wells's 2010 dictionary. Indeed, it made it into the Plena Ilustrita Vortaro by 2002.
The infix "-iĉ-" was another product of the 1970s/1980s. This gives "patro" for parent of unspecified gender, "patriĉo" for father, and "patrino" for mother. It's less mainstream than singular "ge-", but it was popular enough that Jorge Camacho used it in 1991. It's a more logical and consistent system than singular "ge-", but I suspect that resistance to it is partly rooted in the fact that the L1 languages of "Esperantistoj" do not in the majority have unmarked gender-neutrals and marked masculines for similar words. The claim is that it reinterprets a large corpus of existing past Esperanto works, but a counter to that claim is that similar things are currently happening with words like "actor" in English without similar objections.
And, finally, Ido reformed this all the way back in 1907. It went the marked masculine route (of Esperanto's "-iĉ-") with the infix "-ul-".
