Parental Sexuality Squick

Abe: Unsatisfying sex life?
Homer: N -- yes! But please, don't you say that word!
Abe: What, seeeex? What's so unappealing about hearing your elderly father talk about sex? I had seeex.
Homer: [groans]

—The Simpsons, "Grampa vs. Sexual Inadequacy"

Do you have parents? Are your parents human? Do you have one or more siblings who are not your twin/triplet/etc and not adopted?

If you have answered "yes" to any of these questions, then it's entirely likely that your parents have had sex.

Disgusting, innit?

In fiction, it's common enough to see a child react with curiosity to their parents in a Primal Scene. As they get older, however, the very thought that their parents might be getting it on is enough to send them searching for some Brain Bleach.

No real life examples, please; we'd be here all day.

Examples of Parental Sexuality Squick include:

Film - Live Action

Mr. Woodcock: The protagonist sneaks into his mother's house and, when he realizes she's come home, quickly hides under the bed. Guess what she and her boyfriend promptly start doing?

Literature

In Sybil, the titular character is described as having spent her nights until she was about ten in a crib beside her parents' bed, never being given an opportunity to leave the room while they had sex. It's considered among the many, many abuses she endures that causes her disassociation.
In The SM Judge, the protagonists are a judge and his wife. One of his enemies try to destroy his career by publicly using their sex life against him. One tactic used by this enemy is to force their daughter to watch a video of them having BDSM sex. This make her break all contact with her parents. Not because of the scandal or the BDSM, but simply because they had a sex-life, period. "You are my parents, and parents don't have sex!"

Live-Action TV

This makes up roughly 15% of the humor in Cougar Town. In the first episode Travis walks in on his mother performing oral sex on a man his own age, and so the standard is set.
That '70s Show had an episode in which Eric walks in on his parents during sex (unbeknownst to them) and spends the rest of the episode entirely unhinged. It's the only time he ever receives sympathy from his sister. At the end of the episode, Donna tells him of a time when the same thing happened to her.
In an episode of Corner Gas, Brent accidentally records his parents begin to act out a sex fantasy on the gas station's security camera. Afterwards he rips the camera out of the wall and denies that it was ever there.
On The Big Bang Theory, the gang has gotten a hold of one of the prop rings from the The Lord of the Rings movies and are holding it together, with the last one standing getting the ring. Howard and Rajesh try to psych Sheldon out by invoking this about his beloved grandmother.

Howard: Think about it. The only reason you exist is because your grandmother had sex.
Raj: Yeah, Meemaw did the nasty!

In Freaks and Geeks, Sam and Lindsay are horrified when their parents get frisky mid-day... while all their friends are over.
Subverted in one episode of Boy Meets World. Shawn is trying to leave the house quickly and when he walks toward the window he panics because Cory's parents are having fun in the backseat of the car. Instead of being Squicked Cory thinks his parents are really Badass.
Buffy the Vampire Slayer, in 'Earshot'. "You had sex with Giles? You had SEX with GILES??? On the hood of a police car? TWICE?"
In Blossom, her parents nearly sleep together after being divorced for years. Blossom and Tony discuss it afterwards. Tony admits they just like to think their parents had sex only three times (one for each child). Blossom says she likes to think they just did it twice, and Joey was sent to Earth by aliens.
One episode of Arrested Development has Michael convincing his mother to visit his father in a conjugal trailer. While Michael is mildly discomforted by the discussion, his brother GOB reacts with outright disgust when Michael talks to him about it. It Gets Worse when GOB is later detained against the window of said trailer during their visit.

Music

Notably averted in The Lonely Island song "Mother Lover", where a pair of friends played by Andy Samberg and Justin Timberlake go into detail about their plans to seduce the other's single mother as a Mother's Day gift.

"We both love our moms, women with grown women needs"
"I say we break em off, show em how much they really mean"

Newspaper Comics

Two characters in Zits had the stories of their conceptions told to them, with horrible reactions.
- Jeremy was in his room playing "Stairway to Heaven" on his guitar (he always like the song for some reason), when his mother mentions that he probably likes the song so much because he was conceived to it. Within minutes, he's rinsing his ears out and boiling his guitar string.
- Hector's parents let slip that a batch of blueberry muffins burned during his conception. The very sight of a blueberry muffin results in severe nausea.