Sex

Sex was the biological process by which two (or, rarely, more) individuals procreated their species through what was usually regarded culturally as "intimate contact". Since 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.

Sex was also a synonym for gender.

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. (DW: The End of Time) Likewise, Susan self-identified as the grand-daughter of the Doctor (DW: 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. (BFA: An Earthly Child, et al) The Ninth Doctor once seemed distinctly perturbed that Rose Tyler implied he didn't "dance" — a metaphor that had arisen between the two for sex. (DW: 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 "anymore". (DW: Smith and Jones) He also said — long before his daughter was created asexually (DW: The Doctor's Daughter) —that he had been a father. (DW: 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, "Pythia's Curse" had made Gallifreyans effectively sterile. Reproduction was carried out through 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". (NA: Lungbarrow)

The Doctor
Though rare, there were occasional hints that the Doctor did indeed have sex.
 * Benny Summerfield and the Doctor might well have had sex soon after he regenerated into his eighth body. (NA: The Dying Days) Summerfield seemed to confirm the incident occurred when they met again at a later date. (BFA: Benny's Story)
 * The Tenth Doctor entered into a brief but intense romance with Madame de Pompadour, whom he was joyous to have snogged, and heartbroken to have lost. (DW: The Girl in the Fireplace)
 * Before saying goodbye the Doctor asks Rose if she is pregnant, hinting possible intercourse between the two. (DW: Doomsday)
 * The Tenth Doctor apparently had something of an eye for royal women, as he later consummated a wedding to Elizabeth I and enjoyed what was likely a brief marriage to her. After fleeing her — and thereby becoming her mortal enemy — he told Ood Sigma that her nickname "the Virgin Queen" wasn't strictly accurate any longer. The Doctor's sexual involvement with Elizabeth I was later supported by future Queen Liz 10 and by the Dream Lord. (DW: 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 entice the Doctor's eleventh incarnation into having sex with her. Though he refused, at one point they engaged in a snog — which he broke off only after initially kissing her back. (DW: Flesh and Stone) Later, he told Rory Williams, her then-fiancé, that she was a "great kisser", indicating that he had some basis for comparison. (DW: The Vampires of Venice)
 * River Song stated to the Doctor that she was "quite the screamer", then saying "now there's a spoiler for you." Strongly implying a future sexual relationship with the Doctor. (DW: The Impossible Astronaut)

Specific incidents

 * Jan and Ace had rampant sex on Heaven. (NA: Love and War)
 * When Bernice Summerfield excavated Ice Warrior citadels on Mars she made love to a man called Tim in one of the egg chambers. (NA: Lucifer Rising)
 * The inhabitants of Delphon find the surgical removal of limbs to be highly sexually alluring. (NA: Lucifer Rising)
 * After getting to know (a clone of) Abslom Daak, Ace considered having sex with him. (NA: Deceit)
 * Ace had sex with Bill Cook, first mate of the Arrow. (NA: Legacy)
 * Ace had sex with Tom Dekker whilst in Chicago, 1929. (NA: Blood Harvest)
 * Roz Forrester decided in her youth that sex didn't offer anything that Martian Ale didn't. (NA: The Also People)
 * Chris Cwej had sex with a member of The People whilst at the Dyson Sphere. (NA: The Also People)
 * Jason Kane had sex with several aliens while stuck away from Earth. (NA: Death and Diplomacy)
 * Bernice Summerfield claimed to have had the best sex ever with Jason Kane. (NA: Death and Diplomacy)
 * Chris Cwej had sex with Ishtar Hutchings whilst on Earth in 2010. (NA: Happy Endings)
 * In the lead up to Benny's wedding, Ace has sex with a (clone of) Jason Kane. (NA: Happy Endings)
 * Ace lost her virginity to Sabalom Glitz on the floor of his spaceship. (NA: Happy Endings)
 * Chris Cwej had sex with David Daniels while on Earth in 1987. (NA: Damaged Goods)
 * Bernice Summerfield once accidentally had sex with a Citdbtbed. (BNA: Walking to Babylon)
 * Chris, after meeting with the Daleks, has sex with Christine Summerfield, just to feel something human again. (BNA: Dead Romance)
 * Khiste has sex with Christine Summerfield to prove a point about manipulation. (BNA: Dead Romance)
 * Bernice Summerfield's body has sex (whilst under control of Avril Fenman) with Adrian Wall. She fell pregnant with a half human-half Killoran baby. (BFBS: The Squire's Crystal)
 * In one timeline Samantha Jones lost her virginity at 15. (EDA: Alien Bodies)
 * Marie (a Type 103 TARDIS) was mated with a Type 105 on Simia KK98 to produce another TARDIS within her. (EDA: Alien Bodies)
 * Samantha Jones has at least two lovers whilst on Ha'olam. (EDA: Seeing I)
 * Fitz Kreiner has sex with Samantha Jones whilst in San Francisco. (EDA: Unnatural History)
 * Fitz Kreiner has non-committal sex within the structured reality of the Parallel stations. (EDA: Parallel 59)
 * Ace has sex with Kedin Ashar on Mendeb Three. (PDA: Independence Day)
 * Sarah Jane Smith has sex with Tom Ryder in China. (PDA: Bullet Time)
 * A gaseous species was freed from a meteorite that fed on sexual energies, killing several people in Cardiff in the process of its desire for these energies. (TW: Day One)
 * Gwen Cooper had sex with Owen Harper following a disturbing encounter with Torchwood. (TW: Countrycide)
 * Owen Harper had a brief sexual relationship with Diane Holmes who had fallen through the Cardiff rift. (TW: Out of Time)
 * Jack Harkness and Ianto Jones had sex on several occasions, in several areas of Torchwood 3. (TW: Adrift)
 * The Doovari's spacecraft were powered via a 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-meet one another in Cardiff. (TW: 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 became Arnold Golightly. (DW: 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.(DW: Aliens of London),Doctor Who: The Encyclopedia
 * Adam Smith had sex with Toshiko Sato. (TW: Adam)

Other contexts

 * Many Time Lords dreamed of being a different sex (gender). (NA: Human Nature)
 * In one possible timeline, Time Lords who committed suicide regenerated into a body of the opposite sex. (DWU: Exile)
 * Iris Wildthyme confided in Sam and the Doctor that regeneration was treated on Gallifrey the same way that sex was treated on Earth. (EDA: The Scarlet Empress)
 * Jason Kane wrote several xenopornographic novels including Nights of the Perfumed Tentacle based on his sexual encounters with various species. (BNA: Beige Planet Mars)
 * The term 'sexually pair bonded' was used to describe a two beings in a sexual relationship in the 24th century. (NA: Shakedown)