(covers information from several alternate timelines)
"There is no one on Deck 9 Section 12 who doesn't know when you're having intimate relations."
Copulation (or sexual intercourse), also known colloquially as sex, sexual act, intimate contact, or intimate relations, or simply relations or coupling, was a type of personal encounter between the genitals of at least two individuals who may or may not have been in an intimate relationship. It was the biological function of procreation in many certain species that distinguished sexual reproduction from the main other type, asexual reproduction. Multiple slang terms existed, describing the act, included: carnal pleasures, improper relations, making love, or final conquest. Pornography was the portrayal of sexual subject matter for purposes of sexual arousal.
One who had never had sex was said to still have their virginity, whereas one who had not had sex for a long period of time was said to be experiencing a "dry spell". Those who intentionally abstained from sex were said to have taken an oath of celibacy or vow of abstinence, while those who were strongly driven towards sex, almost uncontrollably, were known as sex maniacs.
During T'Pol's briefing before Enterprise NX-01's visit to Rigel X in 2151, Trip Tucker joked that she forgot to warn them about drinking the water. T'Pol assured Tucker that Doctor Phlox wasn't concerned with the food and water on the planet, but added that "he does caution against intimate contact." (ENT: "Broken Bow")
According to Talas, who was attempting to escape her confinement to quarters aboard Enterprise (NX-01) in 2154, she explained to her guard, R. Richards, that "Andorian women are permitted one final conquest before their wedding. I've never had relations with a member of your species. I was hoping you'd indulge me." (ENT: "Babel One")
In an alternate 2370, where Worf and Deanna Troi were married, Data explained to Worf the history of their "romantic affiliation", which included Worf's asking William T. Riker formal permission to court Troi. Upon learning this, Worf inquired, "And then we mated?" Data, however, explained that he was "not privy to the exact details of when, where or how your first coupling took place," before offering to "investigate it". Worf assured him that that was not necessary. (TNG: "Parallels")
When Tom Paris was convicted of murder of Banean Professor Tolen Ren, and he was forced to relive the final moments of the victim's life, over and over, Tuvok questioned the nature of Paris' relationship with Lidell Ren, inquiring whether he was "having improper relations with his wife?" Though not denying it, Paris admitted that "their marriage was over. She told Ren they were finished…" When Tuvok indicated that that did not answer his question, Paris explained that "nothing happened between us. Almost nothing." (VOY: "Ex Post Facto")
When the threat of the Dominion invasion of the Alpha Quadrant loomed in 2373, Quark became concerned with how he would make a living, because "the Jem'Hadar don't eat, don't drink, and they don't have sex. And if that wasn't bad enough, the Founders don't eat and don't drink, and they don't have sex either. Which, between you and me, makes my financial future less than promising." Tora Ziyal, however, attempted to reassure Quark, that "it might not be so bad," stating that, "for all we know the Vorta could be gluttonous, alcoholic sex maniacs." (DS9: "By Inferno's Light")
The same year, it was revealed that the Q had never mated before, as they felt that they were "way beyond sex." Nevertheless, Q and the female Q mated, producing a son nicknamed Junior. (VOY: "The Q and the Grey")
By the 24th century, Starfleet personnel apparently required permission from a CMO and their commanding officers to engage in sexual relations with members of alien species, although this formality was only shown being observed once aboard USS Voyager. (VOY: "Prophecy") Nevertheless, its breach was the basis for a reprimand being entered into Harry Kim's permanent record. When Kathryn Janeway informed Kim of the reprimand, she indicated that the regulations on inter-species sexual contact were "three centimeters thick." (VOY: "The Disease")
After Harry Kim was admitted to sickbay after it was noticed that his "epidermis luminesced" in 2375, he begrudgingly admitted to The Doctor that he had sex with the Varro female Derran Tal. After The Doctor rhetorically asked "You had intimate contact with an alien species without medical clearance?" Kim explained that it was "in the heat of the moment," adding that he didn't think anything would have happened, much less that he would have been affected by olan'vora. (VOY: "The Disease")
People also used holographic programs for sexual gratification, in commercial holosuites or, on starships, the holodecks. (TNG: "The Perfect Mate"; DS9: "The Forsaken", "Meridian", "The Emperor's New Cloak"; VOY: "Blood Fever", "Body and Soul"; LD: "Moist Vessel")
In 2381, while on Frylon IV, Ensigns Brad Boimler and Sam Rutherford hid inside a hollow log from two gumatos who were mating (and a third mugutu who was simply watching the other two). (LD: "Mugato, Gumato")