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 did not consent 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)

Virginity was an individual's initiation into sex. In one timeline, Samantha Jones lost her virginity at 15. (PROSE: Alien Bodies) When Ace was 16 lost her virginity to Sabalom Glitz on the floor of his spaceship after being swept there via a time storm. (PROSE: Happy Endings)

Sex was also a synonym for gender. (PROSE: Human Nature)

Some individuals, such as Roz Forrester) decided in (her youth) that sex didn't offer anything that Martian Ale didn't. (PROSE: The Also People)

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

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)

Bernice Summerfield once accidentally had sex with a Citdbtbed by shaking hands. (PROSE: Walking to Babylon)

Whilst stuck in deep space in the 1980s Jason Kane had sex with several aliens, he eventually turned these experiences into several works of xenopornography including Nights of the Perfumed Tentacle. (PROSE: Death and Diplomacy, Beige Planet Mars)

Several species have disguised themselves as human and then engaged in sex with other humans whilst disguised such as the Zygon Kritakh who had sex with Lauren Anderson, (HOMEVID: Zygon: When Being You Just Isn't Enough) and also member of the Slitheen family disguised as Oliver Charles who around early 2006 enjoyed a number of sexual encounters with Oliver's wife, mistress and a young farmer. (TV: Aliens of London)

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)

TARDISes
Marie (a Type 103 TARDIS) was mated with a Type 105 on Simia KK98 by the Time Lords to produce another TARDIS within her, it was taken away from her shortly after its birth. (PROSE: Alien Bodies)

Sex and psychology
Chris Cwej, after meeting with the Daleks, had sex with Christine Summerfield, just to feel something human again. (PROSE: Dead Romance) Similarly Gwen Cooper had sex with Owen Harper following a disturbing encounter cannibals and Torchwood. (TV: Countrycide)

Khiste had sex with Christine Summerfield to prove a point about manipulation. (PROSE: Dead Romance) Whilst Adam Smith manipulated Toshiko Sato's personality to make her bolder and more assertive and then had sex with. (TV: Adam)

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)

In 2002, Fitz Kreiner had sex with Samantha Jones whilst in San Francisco, Sam agreeing wanting to be with someone who wasn't attempting to manipulate her. (PROSE: Unnatural History)

Attraction
The inhabitants of Delphon found the surgical removal of limbs to be highly sexually alluring. (PROSE: Lucifer Rising)

Procreation
According to Rhys Williams, several power cuts in the 1970s resulted in baby booms, as people "took advantage of the dark". (TV: The Gathering) In 1988, Augustus and Tabetha conceived Amelia Pond.

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)

Inter-species procreation
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)

In the 26th century 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)

Chris Cwej had sex with Ishtar Hutchings whilst on Earth in 2010, following this encounter she became pregnant eventually giving birth to Jasmine Surprise Cwej-Hutchings. Who went on to have sex Ricky McIlveen, they produced a child, an Eternal; Time. (PROSE: 'Happy Endings), Warchild'')

Specific individuals
Several individuals, especially those who travelled with the Seventh and Eighth Doctors had sex with several individuals.

Whilst travelling with the Seventh Doctor Ace had sex with several people including; Kedin Ashar on Mendeb Three and Jan on Heaven. (PROSE: Independence Day, Love and War) Following her departure from travelling with the Doctor and her return having spent time in the Spacefleet she continued with individuals such as; Bill Cook, first mate of the Arrow, (PROSE: Legacy), Tom Dekker whilst in Chicago, 1929. (PROSE: Blood Harvest), a clone of Jason Kane just prior to Bernice Summerfield's wedding (PROSE: Happy Endings). Additionally after getting to know (a clone of) Abslom Daak, Ace considered having sex with him. (PROSE: Deceit)

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)
 * 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)