Sexes

Genders (or sexes) were a type of biological variation between different members of the species. They were key to sexual reproduction, the creation of offspring. In many humanoid species, this involved a combination of two or more individuals' genetic material. It was also possible for members of certain species with compatible or related genetic structures to practice interspecies reproduction.

Certain cultures facilitated this bonding with some form of a wedding ceremony.

In many animal species, the female initiated the mating ritual. Janeway reminded Tom Paris of this when they hyper-evolved into salamander-like species and produced offspring. 

Male and female gender
Many humanoid cultures had a two-sex system based around the division between male and female, or the masculine and the feminine, each of which contained a number of minor anatomical differences. One each of both sex, known as mates, were required to reproduce in a process called sexual reproduction. 

According to James T. Kirk, the "idea of 'male' and 'female' were universal constants." 

According to Counselor Deanna Troi, "Human males are unique. Fathers regard their sons as children, even into adulthood. Sons chafe against their fathers' perceived expectations."

Some more-advanced lifeforms also maintained such distinctions. When two members of the Q Continuum chose to practice two-party reproduction, they manifested themselves in such a manner. However, Q revealed a scornful attitude toward Human females when he discovered Humans in the Delta Quadrant a century sooner than expected: "This is what happens when you put a woman in the captain's chair!"

In 2151, T'Pol queried whether Jonathan Archer would be so determined to locate an apparition were it a "scantily-clad man." 

Gender roles
Upon first being created, a genderless Lal regarded the android's lack of gender as rendering itself "inadequate". Data told it this is why it ought to choose one, to complete its appearance. 

William T. Riker referred to an Earth nursery rhyme, "What Are Little Boys Made Of?", which stated: "Girls are made from sugar and spice, boys are made from snips and snails... and puppy dog tails," to describe the "old-fashioned way of looking at the sexes" to the androgynous Soren. He later clarified that "physically, men are bigger, stronger" and that they "have different sexual organs." He also noted, "Men can't bear young." Worf once explained, "It is my understanding that in most Human families, the woman shares in the cooking."  In Earth's past, it was noted that only landowners of a particular gender had any rights at all. 

The planet Angel I was considered "an unusual matriarchal society" where the female was aggressively dominant, as well as larger and stronger than the male counterpart, in what was considered an arrangement most sensible and natural. 

Vissian cogenitors had the same learning potential as the male and female sexes. However, it was considered "not right" for one to learn to read. 

Other variants of sex and gender
There were also various alien sexual makeups which consisted of different constructs or combinations of sexes:

Androgynous species
Androgyny, also known as being "genderless" or "neuter", was the absence of distinct genders. A number of androgynous species were known, including the Axanar, Xindi-Insectoids, Bynar, and more.

When considering the health of Sluggo, a slug-like creature discovered by Hoshi Sato in 2151, she referred to it as a "she". However, Doctor Phlox questioned the identifier, stating, "We haven't been able to determine its gender yet, if it has one."

Hermaphroditic and transgender species
Hermaphrodism referred to having the characteristics of two or more sexes. Unlike androgynes, hermaphrodites could function as either a male or a female during sex, or undergo a functional transformation from one gender to another.

During the wedding ceremony of William T. Riker and Deanna Troi, Data also welcomed the "invited transgendered species." 

More than two sexes or genders
Some species had more than two sexes, or different capacities were assumed by sexes not conforming to the pattern of male/female bonding which seemed to be common throughout the galaxy.

According to T'Pol, "Tri-gender reproduction [was] not uncommon," while Phlox noted that, "Multi-gendered techniques [for reproduction] aren't always the same." For example, Vissians required three separate sexes to reproduce: male, female, and cogenitor. Phlox speculated that "the cogenitor provides an enzyme which facilitates conception," and it was later revealed by a Vissian that cogenitors were "only needed when a couple's trying to have a child," and made up a small percentage of the overall population, about three percent. 

Species 8472 had as many as five sexes, and Doctor Phlox believed Rigelians to have four or five genders. 

Background information
Kate Mulgrew as Captain Janeway was the first female lead actor of a Star Trek series. The first female captain seen on Star Trek was the Saratoga captain seen in, played by Madge Sinclair. While Majel Barrett's character Number One was the first female first officer to appear in Star Trek, it was Nana Visitor's character, Kira Nerys, that was Star Trek's first female regular character to serve as first officer. The first pairing of a female captain and a female first officer serving aboard the same starship as each other was the USS Shenzhou's Philippa Georgiou and Michael Burnham in and. 

According to and, Andorian marriages typically involved four partners and Bolian marriages might include co-spouses. It was not revealed what, if anything, this said about the genders of the aforementioned species. Only male and female Andorians have been referred to on screen, and no on-screen Andorian child has been referred to as having more than two parents.

In ultimately unused dialogue from the writers' first draft script of, Dr. Phlox tried to reassure Hoshi Sato that his Pyrithian bat bit males only, but Sato wasn't so sure that the bat could tell the difference between genders.

Apocrypha
Several non-canon novels and comics (such as the DS9 series) have portrayed Andorians as having four distinct sexes required for mating. Other novels portray other cultures having three or more sexes, such as Diane Duane's description of a Horta character as being an "orthomale type B4-A".