Sexual desire, sexual arousal, or sexual pleasure was the physical state of desiring, anticipating, or enjoying the pleasure of sex. The inability to physically display sexual arousal was sexual inadequacy known as impotence. (TNG: "Phantasms")
While discussing the use of perfume as an aphrodisiac during a visit to the planet Angel I in 2364, William T. Riker explained to Data (who was unfamiliar with the term), that "an aphrodisiac is something used to stimulate or enhance sexual pleasure." (TNG: "Angel One")
According to Benjamin Sisko, during his first contact with the Bajoran Prophets, that "corporeal entities, [such as] Humans, find physical touch to cause pleasure," like "good feelings, [and] happiness." (DS9: "Emissary")
When Data experienced emotion for the first time in late 2369, during an encounter with the former Borg, he attempted to recreate several scenarios that might evoke further emotions, which included making "four attempts to induce sexual desire by subjecting myself to erotic imagery." (TNG: "Descent")
During Data's holodeck session with Sigmund Freud in 2370, regarding dreams he was having that included imagery of a knife. Freud explained how "the knife in its violent connotation suggests a certain feeling of sexual inadequacy," however, Data protested that he had "no sexual desire." This lead Freud to conclude that Data had "impotence on top of everything." (TNG: "Phantasms")
Beverly Crusher being sexually aroused
When describing her experience with Ronin to Deanna Troi in 2370, Beverly Crusher explained that she felt the presence of a caress, like "a pair of hands […] moving across my skin." She continued, "there was a voice, a man. He whispered my name. It was as if I knew him, or more like he knew me. He knew exactly how I liked to be touched. It was the most physical dream I've ever had. The sensations were very real and extremely arousing." (TNG: "Sub Rosa")
When Seven of Nine downloaded The Doctor's program into her cybernetic matrix, he experienced, for the first time, many physical sensations in 2377. Later when Seven confronted The Doctor about his actions in her body, she pointed out his lack of restraint, notably how he became sexually aroused during a massage received from Jaryn. The Doctor argued back, "when did it become a crime to enjoy a sensation or two?" (VOY: "Body and Soul")
Later that year, when Icheb incorrectly believe that B'Elanna Torres was interested in him for "romantic involvement" in 2375, after she was simply trying to be friendly, he pointed out to the confused Torres that "you've made all the customary overtures: finding reasons to spend time with me, complimenting me, inviting me to engage in recreational activities. Your husband even challenged me to a ritual contest." Even after she explained that he had the wrong idea, he noted that he scanned her, and noted that her "blood pressure and neurotransmitter readings indicated a state of arousal." Embarrassed, she explained that they were in a Jefferies tube, "it was hot, and I've been under a lot of pressure, but I certainly wasn't aroused." (VOY: "Nightingale")
Still later that year, following USS Voyager's acquisition of the crew from Kohlar's battle cruiser force the crew to bunk up with a number of Klingons, Harry Kim came into sickbay with a bite mark, complaining that he was attacked by a Klingon. When The Doctor asked what Kim did to provoke him, Kim explained, "not him, her. And she wasn't provoked. She was aroused." The Doctor took that into consideration and said that he wasn't surprised, as he had studied the section on Klingon mating rituals in their cultural database.
When Kim asked how he should convince a female twice his size that he wasn't interested, The Doctor said that he probably couldn't, and told Kim he basically had two options: "Kill her, or mate with her." Seeing as "the first option is clearly unacceptable," he gave Kim authorization "to engage in intimate relations with a member of an alien species," adding that he should "be sure to get the Captain's approval as well." Later, when Kim was trying to avoid the Klingon, he explained his situation to Neelix, who himself found the Klingon in question, Officer Ch'Rega, to be "a fine specimen of Klingon womanhood," before offering to take her of Kim's hands. (VOY: "Prophecy")
In 2380, during an away mission on Gelrak V, Ensign Beckett Mariner found herself aroused while watching Commander Jack Ransom fight the Gelrakian champion, Vindor. (LD: "Temporal Edict") Sometime later, Ensigns Mariner and Brad Boimler discovered a parasite that made its host “chemically irresistible to potential mates,” which was the catalyst for the latter's relationship with Lieutenant Barbara Brinson. (LD: "Cupid's Errant Arrow") Later that year, during an interview with Captain Carol Freeman, Boimler blurted out that his fellow ensign Mariner is “hot,” irritating the captain and prompting the ensign to backtrack on his previous statement before running out of the room. (LD: "Crisis Point")