Maybe it's the lower air pressure. Maybe it's travelling at 500 mph. Maybe it's just a desire to do what few people have ever done. The complimentary wine probably plays a part. It's unclear, but sometimes people just like to have sex on aircraft. Usually in the toilet, which is not exactly the most romantic location on the planet (nor spacious on most airplanes), but is one of the few private places.
The Other Wiki has an article on this. It is worth noting that the airline company will probably sue you, since it's of questionable legality. There are, however, dedicated companies for this sort of thing.
For the stage beyond the Mile High Club, see Zero-G Spot. Compare Three-Way Sex in terms of sexual rubicons all men are supposed to aspire to.
In Weekend at Hisao's, Hisao and Shizune, flying back from Lilly's wedding, have sex in the on-board lavatory. Hisao complains that it's too cramped to enjoy himself, and they get caught by the stewardess.
Discussed in many adult-themed MLPFIM fics. While using clouds is a common practice among pegasi, actually doing it in flight is usually ridiculed - since in fanon pegasi wings are susseptible to Something Else Also Rises, it quickly becomes impossible to fly properly.
In the Steven Universe fanfic Aerial Choreography, Steven and Amethyst play with this trope without a plane. While they don't actually have sex, their hormones from passionately kissing each other become powerful enough to start a FOREST FIRE.
Airplane II: The Sequel. A woman seduces a man by saying she doesn't want to die a virgin. Later, she tries the same trick again with another man, and a number of men are shown lined up waiting to have sex with her. At one point, she does the same thing with a donkey.
Red Eye. While Rippner locks himself and Lisa in the bathroom and gives her a smackdown, the flight attendants assume that they're having sex. In fact, the sexual subtext makes the scene all the more disturbing. Though he walks out of there perfectly composed and smug, she's terrified and nearly in tears—and it's only made more creepy when you find out that Lisa was raped by a Knife Nut like Jackson before the two even met.
1941. Ice Queen Donna Stratton turns into a raging nymphomaniac whenever she gets into the air. Knowing this, Handsome Lech Captain Loomis Birkhead is willing to risk death (he flunked flying school) to get her up in an aircraft. Just as he's finally mastered the plane (and her) they're interrupted in the act by a mad pilot in a P-40 Warhawk who thinks they're Japs.
Frequent-flyers Ryan and Alex compare Mile High Club notes in Up in the Air.
Jacqueline Bisset picks up a guy and gets it on with him in a plane lavatory (while the plane is coming in for a landing) in her 1981 movie Rich and Famous.
In Bridesmaids, Megan attempts to seduce a man (whom she thinks is an undercover air marshal) into doing this. He declines. Presumably because it turns out he really is an air marshal.
In the obscure mid-1980's softcore movie Love Circles Around The World, a pilot has sex with a stewardess in the cockpit in the middle of a flight. The couple joggles various instruments, causing the plane to bounce around alarmingly, and the stewardess hits the intercom switch with her high heel at a crucial moment, causing the sounds of the proceedings to be broadcast into the passenger cabin.
James convinces a stewardess into doing this in Rush, but in this case it's less about fanservice and more about showing James's increasing restlessness and frustration with life, given it happens in the middle of (at the time) a bad racing season for him and right after his very public divorce in which his wife left him for Richard Burton.
One of the subplots in Small World by David Lodge involves the ridiculously competitive Howard Ringbaum trying to convince his wife Thelma to have sex with him on an aeroplane so he can join the Mile High Club, apparently labouring under the misaprehension that the Mile High Club is an actual organisation.
Joked about in Spin City, where Mike and Caitlin simply talk in the airplane's bathroom. When they leave, the other passengers assume they have joined the mile high club, and the men proceed to give Mike high fives.
Friends. Monica and Chandler plan to meet in the bathroom on their flight back to New York from London, but Joey thwarts them by striking up a conversation with Chandler.
A Leverage episode entitled The Mile High Job averts this. No sex on a plane, although there were plenty of other shenanigans.
Danny Blue from Hustle with two airline stewardesses. In "Gold Mine" he makes a remark about a soap dispenser almost putting out a stewardess's eye.
Drs. Rodney McKay and Jennifer Keller are implied to have joined in a private jet at the end of the Stargate Atlantis episode "Brain Storm."
Married... with Children. While skydiving, Bud's date Kara has sex with Biff, the skydiving instructor after Bud's pushed out of the plane in "The Mystery of Skull Island."
The MTV show Fur TV had the Handsome Lech (well, a sock puppet, but all the ladies treat him as such) propose this to whomever happens to have a hundred million dollars. "A plane filled with woman catering to your every need, 30 000 feet above any legal restrictions. I call it 'The Flying Fuck'."
It is reported in Red Dwarf that an alternative, wealthy version of Dave Lister would meet his end at the age of 98 in a plane crash... caused by him losing control of the plane while having sex with his fourteenth wife.
This might result in death, as shown in 1000 Ways To Die. The couple are having sex in the airplane lavatory until heavy turbulence tosses them in the confined space, cracking their skulls and killing them both.
Referred to in an episode of Frasier: When Niles was young he went to Paris, and a woman on the plane asked if he'd like to join the Mile-High Club. He didn't want to admit he was unfamiliar with the term, so he said he didn't fly often enough for that to be worthwhile.
In one episode of Cheers Carla claimed she did this with Nick on their honeymoon; unfortunately, the plane experienced some rather heavy turbulence when they did, and it scared her to death, giving her a rather bad fear of flying from then on.
Rowan: I met her in the first class lounge of a jumbo jet It was love at first sight, Romeo and Juliet Kate: He looked pretty rich and I was down on m' luck So I charged him a fortune for a flying fu... Rowan:...for crying out loud!
International Lover, the final track on Prince's 1999 album.
Ron White joked that instead of the "Mile High Club", he's a member of the "Mile Ahead Club", which is where you do it behind a Cracker Barrel billboard.
A weird example in Metal Gear Solid 4 has Naomi and Otacon having sex, in a stationary helicopter, inside a flying plane. Er?
Just Cause 2 has a literal mile high club: an aircraft carrier supported by two blimps with dancing girls and evil bartenders.
The main character of the eroge Monmusu Quest can meet his demise from being raped while flying over the ocean. In the Game Over scene, the narrator asks him if it was worth joining the "exclusive club".
Alter Ego has this crop up late in life, sometimes, where your significant other has a pilot's license and private plane.
An achievement for beating the Bonus Level of Call of Duty 4: Modern Warfare on the highest difficulty references the practice by being named this. The level actually consists of quickly assaulting the enemies inside an airplane and rescuing a hostage in a game with pretty much no other mentions of sexuality, which makes the barely-connected name rather Getting Crap Past the Radar.
In a guest strip (August 2 2010, between #333 and #334, NSFW), Dillon claims to have joined it ... in the cockpit.
Senna and Gary have sex on a private jet en route to Paris (Strip #780, August 20th 2013, NSFW funnily enough). Gary asks if they've joined the club; the answer would be "yes". Although they're eventually interrupted, penetration had occurred, and although the aircraft is on its landing approach by then, they must have been at least a mile high when they started.
In a Halloween episode of the The Simpsons, Kang and Kodos select Marge for their "cross-breeding program".
"To put you at ease, we have recreated the most common spawning locations of your species. You may choose either: the back seat of a Camaro, an airplane bathroom, a friend's wedding, or the alley behind a porno theatre."
There is a Fun T-Shirt that said "Million Mile High Club" and had a picture of astronauts having sex. It should be pointed out that the "highest" any astronaut has flown is roughly 300,000 miles, the distance to the Moon, and no woman has left Low Earth Orbit (700 miles), but "Three-Hundred-Thousand Mile High Club" just doesn't have the same ring to it.