Sex

Sex was the action by which two (or sometimes more) individuals engaged in what was frequently regarded culturally as "intimate contact." Often, this was the means by which they procreated their species. However, as sex was generally associated with positive emotional and physical responses — except when one or more parties didn't agree to it, as during rape — sex could also be practised for recreational, and not strictly procreational, ends.

Indeed, sex could be practised between two or more individuals of the same gender, which rarely had any procreative implications at all. Precise definition on what specific actions constituted sex varied by species and culture. By the 21st century, humans did not need to have sex to reproduce — the males donated sperm and it was injected into the females by syringe. (TV: Out of Time)

Sex was also a synonym for gender.

Species
According to the Eleventh Doctor, sharks, birds, bees, and even educated flees had sex. (TV: Hide)

Time Lords
How Time Lords related sexually was unclear.

By most accounts, Time Lords were implied capable of sexual procreation. The Master, for instance, claimed to have a father. (TV: The End of Time) Likewise, Susan said she was the grand-daughter of the Doctor (TV: An Unearthly Child, et al) and she had at least one child — whom she identified as the Doctor's great-grandson —by the human David Campbell. (AUDIO: An Earthly Child, et al) The Ninth Doctor once seemed distinctly perturbed when Rose Tyler implied he didn't "dance" — a metaphor for sex that had arisen between the two. (TV: The Doctor Dances) The Tenth Doctor also described the possibility of genetically collateral relationships when he told Martha Jones he didn't have a brother "any more". (TV: Smith and Jones) He also said that — long before his daughter was created asexually (TV: The Doctor's Daughter) — he had been a father. (TV: Fear Her) Indeed the presence of obvious sexual attraction between the Meta-Crisis Tenth Doctor and Rose Tyler suggested that natural, biological reproduction — or at the very least recreational sex — was possible. "900 years old, me. I've been around a bit. I think you can assume at some point I've 'danced'."

- Ninth Doctor

However, another account suggested that sex had ceased to have a reproductive function on Gallifrey long before the Doctor's time. According to this school of thought, the "Pythia's Curse" had made Gallifreyans effectively sterile. Reproduction was carried out by the carefully controlled use of Looms, which produced new Time Lords as fully-formed adults. No direct genetic relationships were therefore possible. Time Lords arising from the same House — for each House had only one Loom — were considered "cousins". (PROSE: Lungbarrow)

The Doctor
Though rare, there were occasional hints that the Doctor did indeed have sex.
 * Benny Summerfield and the Doctor might have had sex soon after he regenerated into his eighth body. (PROSE: The Dying Days) Summerfield seemed to confirm the incident when they met again at a later date. (AUDIO: Benny's Story)
 * The Tenth Doctor entered into a brief but intense romance with Madame de Pompadour, whom he was joyous to have kissed, and heartbroken to have lost. (TV: The Girl in the Fireplace)
 * The Tenth Doctor apparently had something of an eye for royal women, as he later implied having consummated a wedding to Elizabeth I after a (presumably brief) marriage to her. After fleeing her — and thereby becoming her mortal enemy — he stopped just short of telling Ood Sigma that her nickname, "the Virgin Queen", was no longer accurate. The Doctor's sexual involvement with Elizabeth I was later hinted at by future Queen Liz 10 and by the Dream Lord. (TV: The Shakespeare Code, The End of Time, The Beast Below, Amy's Choice)
 * Amy Pond, following a near-death experience with the Weeping Angels, attempted to seduce the Doctor's eleventh incarnation. Though he refused, he implied in answer to her question that he hadn't had sex in "a while". (TV: Flesh and Stone)
 * River Song intimated on several occasions that she and the Eleventh Doctor had (or will have, from his perspective) a sexual relationship. (TV: The Impossible Astronaut) Their eventual marriage seemed to support this as well; as the Doctor claimed to Dorium Maldovar that River's nights spent in prison were between her and himself. (TV: The Wedding of River Song)

Specific incidents

 * James Stevens once had a one night stand with a girl named Natasha, much to the dismay of his wife, also named Natasha, who left him. (PROSE: Who Killed Kennedy)
 * Jan and Ace had rampant sex on Heaven. (PROSE: Love and War)
 * When Bernice Summerfield excavated Ice Warrior citadels on Mars, she had sex with a man called Tim in one of the egg chambers. (PROSE: Lucifer Rising)
 * The inhabitants of Delphon found the surgical removal of limbs to be highly sexually alluring. (PROSE: Lucifer Rising)
 * After getting to know (a clone of) Abslom Daak, Ace considered having sex with him. (PROSE: Deceit)
 * Ace had sex with Bill Cook, first mate of the Arrow. (PROSE: Legacy)
 * Ace had sex with Tom Dekker whilst in Chicago, 1929. (PROSE: Blood Harvest)
 * Roz Forrester decided in her youth that sex didn't offer anything that Martian Ale didn't. (PROSE: The Also People)
 * Chris Cwej had sex with a member of The People whilst at the Dyson Sphere. (PROSE: The Also People)
 * Jason Kane had sex with several aliens while stuck away from Earth. (PROSE: Death and Diplomacy)
 * Bernice Summerfield claimed to have had the best sex ever with Jason Kane. (PROSE: Death and Diplomacy)
 * Chris Cwej had sex with Ishtar Hutchings whilst on Earth in 2010. (PROSE: Happy Endings)
 * In the lead up to Benny's wedding, Ace has sex with (a clone of) Jason Kane. (PROSE: Happy Endings)
 * Ace lost her virginity to Sabalom Glitz on the floor of his spaceship. (PROSE: Happy Endings)
 * Chris Cwej had sex with David Daniels while on Earth in 1987. (PROSE: Damaged Goods)
 * Bernice Summerfield once accidentally had sex with a Citdbtbed. (PROSE: Walking to Babylon)
 * Chris, after meeting with the Daleks, had sex with Christine Summerfield, just to feel something human again. (PROSE: Dead Romance)
 * Khiste has sex with Christine Summerfield to prove a point about manipulation. (PROSE: Dead Romance)
 * Bernice Summerfield's body had sex (whilst under control of Avril Fenman) with Adrian Wall. She fell pregnant with a half human-half Killoran baby. (PROSE: The Squire's Crystal)
 * In one timeline, Samantha Jones lost her virginity at fifteen. (PROSE: Alien Bodies)
 * Marie (a Type 103 TARDIS) was mated with a Type 105 on Simia KK98 to produce another TARDIS within her. (PROSE: Alien Bodies)
 * Samantha Jones had at least two lovers whilst on Ha'olam. (PROSE: Seeing I)
 * Fitz Kreiner had sex with Samantha Jones whilst in San Francisco. (PROSE: Unnatural History)
 * Fitz Kreiner had non-committal sex within the structured reality of the Parallel stations. (PROSE: Parallel 59)
 * Lauren Anderson had sex with the Zygon Kritakh. (HOMEVID: Zygon: When Being You Just Isn't Enough)
 * Ace had sex with Kedin Ashar on Mendeb Three. (PROSE: Independence Day)
 * Sarah Jane Smith had sex with Tom Ryder in China. (PROSE: Bullet Time)
 * In 1988, Augustus and Tabetha conceived Amelia Pond.
 * A gaseous species that fed on sexual energies was freed from a meteorite. It killed several people in Cardiff in the process of its desire for these energies. (TV: Day One)
 * Gwen Cooper had sex with Owen Harper following a disturbing encounter with Torchwood. (TV: Countrycide)
 * Owen Harper had a brief sexual relationship with Diane Holmes, who had fallen through the Cardiff rift. (TV: Out of Time)
 * Jack Harkness and Ianto Jones had sex on several occasions, in several areas of Torchwood 3. (TV: Adrift)
 * The Doovari's spacecraft were powered by an energy drive that was derived from the energy produced during sex. (WEB: Torchwood.co.uk)
 * Jack Harkness is known to have had sex with many different people and species.
 * Jack Harkness and John Hart allude to a sexual relationship when they re-met one another in Cardiff. (TV: Kiss Kiss, Bang Bang)
 * In 1886 Clemency Eddison fell in love with a Vespiform who had taken the form of a human. She had sex with him and fell pregnant; the resulting child was Arnold Golightly. (TV: The Unicorn and the Wasp)
 * Around early 2006 a member of the Slitheen family disguised as Oliver Charles enjoyed a number of sexual encounters with Oliver's wife, mistress and a young farmer. (TV: Aliens of London) (REF: Doctor Who: The Encyclopedia)
 * Adam Smith had sex with Toshiko Sato. (TV: Adam)
 * After Ursula Blake was reduced to a face in a paving slab after she was absorbed by the Abzorbaloff, Elton Pope said they still had "a bit of a love life", implying oral sex. (TV: Love & Monsters)
 * Amy Pond and Rory Williams had sex on board the TARDIS on their wedding night, resulting in the conception of Melody Pond. (TV: A Good Man Goes to War)
 * According to Rhys Williams, several power cuts in the 1970s resulted in baby booms, as people "took advantage of the dark". (TV: The Gathering)

Other contexts

 * Many Time Lords dreamed of being a different sex (gender). (PROSE: Human Nature)
 * Iris Wildthyme confided to Sam and the Eighth Doctor that regeneration was treated on Gallifrey the same way that sex was treated on Earth. (PROSE: The Scarlet Empress)
 * Jason Kane wrote several xenopornographic novels including Nights of the Perfumed Tentacle based on his sexual encounters with various species. (PROSE: Beige Planet Mars)
 * The term 'sexually pair bonded' was used to describe a two beings in a sexual relationship in the 24th century. (PROSE: Shakedown)
 * The term 'sexy' was used by an individual to describe another who they thought was sexually attractive.
 * In his eleventh incarnation, the Doctor called his TARDIS sexy, but presumably wasn't actually sexually attracted to it. (TV: The Doctor's Wife)
 * The Doctor referred to the Saturnyns as "sexy fish vampires". (TV: A Good Man Goes to War)
 * When Plark told his subordinate to make their simulation of Joseph Serf smile at Sarah Jane Smith, the Skullion made him give a "sexy smile", only to be told by Plark to give him a "thoughtful smile, not sexy smile". (TV: The Man Who Never Was)