Sexual reproduction
Sexual reproduction was the processes by which DNA was passed on to offspring from more than one parent (called mates), via creation of new DNA from combined RNA from both parents. Interspecies reproduction was possible, but might require medical assistance.

The act of sexual reproduction typically involved , which hinged on the usage of sexual organs which, in most species, were differentiated for males and females. 

Alternatively, DNA extracted from a male could be used to impregnate a female without the use of the sexual organs. ()

During the analysis of the Vaalians, and their lack of "replacements", Captain James T. Kirk looked to Spock to answer Yeoman Martha Landon's attempt to understand what would happen if one of them died, upsetting the balance of the population. When looking for a means to find such a replacement, Spock gave the opinion that he saw no alternative. Landon continued, "But these people, I mean, if they don't know anything about. What I mean is, they don't seem to have any natural er. I mean, how is it, done?" Kirk further pressured Spock for an answer, citing "you're the science officer. Why don't you explain it to the young lady." Spock awkwardly explained, "Well, I believe it's safe… safe to assume that they would receive the necessary instructions." Doctor Leonard McCoy, who was also present for the discussion, then scoffed, "from a machine? That I'd like to see." 

Later, following the destruction of Vaal, the topic came up once again, Kirk explained to Akuta that "you'll learn something about men and women, the way they're supposed to be. Caring for each other, being happy with each other, being good to each other. That's what we call love. You'll like that, too, a lot. You and your children." Following the Vaalians puzzlement to Kirk's comment, he added, "just go on the way you're going. You'll find out." ()

Sexual reproduction processes by species

Arkarian horn fowl
Arkarian horn fowl, native to the planet Arkaria, had mating habits of particular interest to ornithologists. Commander Calvin Hutchinson found the practice of particular interest. ()

Borg
While speaking of copulation, Seven of Nine noted that the Borg opted for assimilation and did not require sexual reproduction of any kind, specifying single cell reproduction and seduction. () Additionally, upon encountering a fetal Borg drone in an advanced maturation chamber that had assimilated 29th-century technology from The Doctor's mobile emitter, Seven of Nine said "I don't understand. The Borg assimilate. They do not reproduce in this fashion." ()

Humans
In the Human species, the males and females had different sexual organs. The male inseminated the female, who in turn carried the baby. Commander William T. Riker commented that the act of Human reproduction brings a closeness and intimacy between the man and woman, and is "a very pleasurable experience." ()

J'naii
The J'naii species sexually reproduced by incubating fetuses located in fibrous husks which both parents inseminated. The J'naii considered this method of reproduction less risky and less painful than the processes used by other species, though Soren claimed the process was extremely pleasurable. ()

Klingons
See Klingon mating rituals.

Ocampa
See Elogium.

Risians
See Jamaharon.

Taresians
The Taresian species' process of sexual reproduction (joining) removed so much DNA from the male that it resulted in the male's death. To sustain their numbers, Taresian women lured unsuspecting males from different alien species to their homeworld. ()

Varro
Varro traditionally mated for life. When two Varro had intimate relations, they experienced olan'vora - "the shared heart". This made their body chemistry interdependent, causing withdrawal symptoms if the two people separated. The effect got stronger with each mating and separation could be fatal. ()

Vulcans
See Pon farr.

Wisp (ancestors)
The corporeal ancestors of the Wisps required mates to reproduce (like Humans), but eventually evolved past this requirement. In 2152, Wisps inhabited several crew members of , claiming they wanted to learn how their ancestors lived. ()

See also 
Asexual reproduction

External link 
 
 Sex and reproduction