[[caption-width-right:259:From left to right: Kaho, Yuki, Female Yuki and Natsu]]

Cult classic manga by Atsushi Kamijou that was serialized between 1988 and 1992. Contrary to the title, it contains no sexual intercourse at all, albeit not without fanservice. In fact, "sex" here refers to "gender."

Taking place in Japan during TheEighties, the story follows Kaho, a not so average high school girl who, when her class goes on a school trip to Okinawa, seizes the opportunity to look for her childhood friend, Natsu, whom she hasn't seen in six years. However, instead of Natsu she finds a strange young man named Yuki, a petty criminal who takes her for a ride in a stolen car, gets her involved in a gang fight and almost gets her killed. As it turns out, he's Natsu's friend, and the two of them seem to be involved with gangs and live from smuggling and selling stuff in the black market. Kaho is not one to be easily intimidated, though, and the manga chronicles the trio's adventures in Okinawa and Fussa, involving high school delinquents, weirdos, heists, the US army, the Korean maffia, yakuza, even ''more'' yakuza, and a shady assassin.

With its stylish artwork, movie-like atmosphere and quirky, charismatic characters, ''SEX'' is often regarded as Atsushi Kamijou's best work.

!!The manga provides examples of:
* ArtisticAge: Natsu and Kaho are only sixteen. Yuki may or may not be this case as his age is unknown and can be anywhere between twenty and thirty (what we know of his past suggests that he's definitely over 20).
* ArtShift: The artwork often shifts into a more realistic style.
* AuthorAppeal: Kamijou obviously has a thing for high heels. And he's a butt person.
* ButtMonkey: Chiaki.
* ColorBlindConfusion: Yuki is red-green colorblind, which means he pays no attention to traffic signals.
* ComicTrio: Yuki and Kaho are the weird ones, Natsu is the StraightMan.
* DistaffCounterpart: Female Yuki. (Not without a reason.)
* DistressedDude: [[spoiler:Natsu]] in the last volume. He's not particularly distressed about it, though, until his kidnapper threatens to kill him.
* DrivesLikeCrazy: Yuki. It doesn't help that he's colorblind and can't tell red and green apart.
** CaptainCrash: He has a tendency to crash the vehicles he drives. (He never claimed to be a good driver, though.)
* TheEighties ...in Japan.
** The 'dos and the outfits are probably the only things about the manga that haven't aged well.
* MrFanservice: Yuki, Natsu and Higa... but especially Yuki.
* EvenTheGirlsWantHer: Kaho - detailed in the LesYay section.
* FemmeFatale: Kaho is not quite there yet, but she has great potential to grow up to be one, hampered only by her laid-back and cheerful disposition, and her tendency to be downright weird.
* GratuitousEnglish: By the American soldiers. Also Yuki whenever he talks in English.
* HelloNurse: Kaho manages to have this effect on an entire small town.
* IdentityAmnesia: Yuki.
* InvisibleParents: Kaho's and Natsu's parents are sometimes mentioned but they don't ever show up and are not involved in the story (only as invisible plot devices).
* JapaneseDelinquents: A bunch at Natsu's school. Natsu himself is a delinquent as well, but he doesn't display the stereotypical traits except for bleaching his hair.
* KatanasAreJustBetter: Higa's modus operandi. Sort of justified since he's a {{Yakuza}} miniboss.
* KavorkaMan: Women flock to Chiaki, for some reason.
* Les Yay: In spades between Kaho and Female Yuki who is heavily implied to be in love with Kaho.
** Also present between Kaho and Chigira.
** And one of Kaho's female school mates is very much attracted to her in a one-sided RomanticTwo-GirlFriendship way.
* LongHairedPrettyBoy: Higa.
** He's also a DarkSkinnedBlond (though his dark skin is just a tan).
* MeaningfulName: 'Natsu' means 'summer' which is one of the main themes of the manga.
* MadBomber: Natsu is not particularly unstable (in fact he's the StraightMan of the trio), but he has an affinity towards bombs and other explosives.
* MaleGaze: In spades.
* MsFanservice: Female Yuki is this(although due to the art style she often comes off looking pretty masculine). Kaho becomes this in later volumes.
* MistakenForPregnant: When Kaho gets sick due to PTSD Meg Brown assumes she's pregnant with Natsu's child.
* MobWar: A plot point.
* MurderInc: Mother & Children.
* MysteriousPast: Yuki.
* OnlyOneName: Yuki and Female Yuki. [[spoiler:Turns out Yuki unconsciously took his name from Female Yuki, so his real name is unknown.]]
* OnlySaneMan: Natsu is pretty often this.
* ProfessionalKiller: Female Yuki is an assassin. Also, the yakuza hitmen.
%%* RuleOfCool
* ScarsAreForever: Natsu has scars all over his body.
* ShirtlessScene: Yuki, pretty often. Natsu and Higa get some scenes as well.
* ShoutOut: All over the place. Some examples:
** Yuki and Natsu are based on Harry and Ranmaru, members of the classic Japanese rock band The Street Sliders. (Vocalist and lead guitarist, respectively.)
** [[http://i54.tinypic.com/243ru3t.jpg The cover of volume 5]] riffs on the poster of ''Film/EnterTheDragon''.
** [[Music/TheBeachBoys ''And keepin' the summer alive...'']]
* ShowerOfAngst: Yuki, after Natsu is shot and captured.
* StormingTheCastle: Yuki and Higa when they [[spoiler:try to get Natsu back]].
* {{Stripperific}}: Female Yuki's combat outfits usually combine this with CombatStilettos and VaporWear.
* TeamPet: Susie Q, the leguan.
* TriangRelations: Type 8, sort of, in the trio. Natsu and Kaho clearly have feelings for each other, and everyone assumes they're an item; but there's also open attraction and flirting between Yuki and Kaho. Natsu is also very attached to Yuki to the point of being jealous of Higa, and Yuki regards both Natsu and Kaho as belonging to him.
* TrueCompanions: Yuki, Natsu and Kaho.
* UnluckyChildhoodFriend: Chiaki is implied to be this to Kaho. They get on pretty well, regardless.
* UnusualEyebrows: For some reason Natsu has no eyebrows (he probably shaves them because from time to time he displays some).
* {{Yakuza}}: A ''lot'' of them, including Higa and his gang. [[spoiler:Yuki used to be one, too, but quit to be with Natsu.]]
* YouNeedToGetLaid: Higa's reaction to Natsu's hotheaded behavior. "How about getting some of that energy out of your system? With Kaho."