One of the things most criticised about Esperanto (and certainly one of my most disliked things about it) is its severe gender asymmetry. Traditionally, Esperanto nouns for people were presumed to be masculine by default, with an extra suffix being required to make it feminine. For example, doktoro would be presumed to be a male doctor, with doktorino being required to refer to a female doctor. In modern times this is less the case; most Esperantists would consider doktoro to not specify gender any more. However, there are still some words (especially family words like patro, avo, onklo, frato, as well as knabo “boy” and viro “man”) which are inherently masculine, with the feminine counterparts being derived from the masculine root. There aren’t gender-neutral singular words like “parent” or “sibling” in normative Esperanto. Furthermore, while you can still specify, say, “a female friend” by adding -in- to amiko, you cannot specify “a male friend” the same way. Esperanto also lacks a gender-neutral singular 3rd person pronoun, with the traditional response being, “Just use ĝi ‘it’.”
Numerous Esperantists have developed and employed strategies to deal with these problems. Not all other Esperantists support those strategies; a number insist that the language was perfect the way L.L. Zamenhof created it and that any changes sully the whole thing. But, especially in the modern era, others are more flexible.
One strategy to deal with the lack of gender-neutral words for “parent” and “sibling” is to backform it from the plurals gepatroj and gefratoj. (Obviously, this also goes for all other words in this category.) Esperanto purists don’t like this because the prefix ge- means “both sexes together”.
Then, another strategy to cope with the lack of any masculinising suffix is to invent one. The neater, but more controversial, proposal is -iĉ-. This suffix is chosen because of the words paĉjo “daddy” and panjo “mummy”; the ⟨n⟩ in panjo is seen as related to the ⟨n⟩ in the suffix -in-, so -iĉ- is created by analogy. Anyway, when you use this paradigm, patro comes to mean “parent”, and you’d need patriĉo to say “father”. Esperanto purists hate this because “it’s kontraŭfundamenta to invent new affixes”, and also they don’t like reanalysing the original noun to be gender-neutral.
If you have to specify a noun is masculine, Esperanto purists would prefer you create a compound word by adding vir- (man) to the front of the other word. Esperanto has traditionally done this for a long time for male animals, e.g. virbovo (“bull”, literally “man-cow”). The problems with this are that it’s ugly and that it doesn’t help with the lack of gender-neutral words.
So then, we move on to the problem of the lack of any gender-neutral 3rd person singular pronoun. Now, L.L. Zamenhof’s opinion was that ĝi, usually translated to “it”, was perfectly fine as a pronoun for people of unknown gender. However, most people are not keen to be described as “it” and so this is not a satisfactory solution for most people. Li “he” has also been used to refer to people whose gender is unknown, like it historically has in English, but now people are more enlightened we don’t like presuming everyone is male until proven otherwise either. It is possible to use tiu “that person” in many circumstances to avoid having to specify a gender, but this is sometimes not workable inside subclauses.
An alternative solution is to add an extra pronoun. The most popular proposal is to add ri as a gender-neutral pronoun. Less popular, but still in existence, is ŝli (which is derived from ŝ/li, like English “s/he”). The use of the more popular option ri is known in Esperanto as riismo. It is widely used by young Esperantists in particular, and not even the Akademio de Esperanto criticises it, although it hasn’t adopted it as part of the official standard either. The Plena Manlibro de Esperanta Gramatiko, a respected authority on Esperanto grammar, used to recommend against using ri, but since 2019 no longer has.
My own personal solution to this conundrum is to not speak Esperanto, but Ido 🙂 Ido decided to become fully gender-neutral by default in 1922. A limited list of Ido words (four exactly: viro “man”, muliero “woman”, matro “mother”, patro “father”) are gendered, although non-gendered counterparts (e.g. genitoro “parent”) exist as well. All other words are gender-neutral, and if you want to make them gendered you add either the masculinising -ul- or feminising -in-. It also has an epicene 3rd person singular pronoun, lu, which can be used in preference to the gendered ilu “he/him”, elu “she/her” and olu “it”. In some ways, Ido was the original gender-reformed Esperanto!