Sex

Sex was a process through which two or more individuals proceed through a process of sexual gratification.

The term 'sexually pair bonded' was used to describe a two beings in a sexual relationship in the 24th century. (NA: Shakedown)

Sex was also used to describe the gender of a particular species.

References

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 has sex with Bill Cook, first mate of the Arrow. (NA: Legacy)
Ace has sex with Thomas 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)
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)
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)

Other contexts

Many Time Lords dream of being a different sex (gender). (NA: Human Nature)
Iris Wildthyme confided in Sam and the Doctor that regeneration is treated on Gallifrey the same way that sex is treated on Earth. (EDA: The Scarlet Empress)
Jason Kane wrote several xenopornography novels including Nights of the Perfumed Tentacle based on his sexual encounters with various species. (BNA: Beige Planet Mars)