She was horny because she was an inexperienced virgin who spent all her life studying instead of dating.
This made her awkwardly tense around everypony she ever talked to. Her friends, Princess Celestia, Shining Armor, her mother and father, and Flash Sentry. She fantasized about all of them.
But she knew she didn't stand a chance with any of them.
She never asked any of them out, because they would think of her as a pathetic loser when they find out she's a virgin.
Twilight wrote down her sexual fantasies about herself, rutting her friends and family, as a series of novels, published under a pseudonym.
They were the bestselling books in all of Equestria. They dominated the sales charts.
Other writers imitated this and wrote their own smut novels about Twilight Sparkle having sex. The fad made most other books unprofitable, include many of Twilight's favorites. Those writers eventually got frustrated and quit the business.
Having hardly any new books to read made Twilight Sparkle sad.
And despite all the literature about her fictional sex life, Twilight was no less horny than before.
She was constantly sleeping with her friends. Rarity, Rainbow Dash, Pinkie Pie, and sometimes even Fluttershy.
She would be open to sleeping with Twilight too, if she ever asked. But Twilight never did.
One of them would visit on any given day. They would flirt during Applejack's farmwork until she gives in and takes a break. Then they'd find a private spot.
Applejack didn't know why, but her friends were all madly in love with her rustic country charm. Or perhaps it was her strong, toned body. Or her blonde mane color. Her homemade apple-based snacks probably helped too.
The sex was always great. Then there would always be some argument over a trivial issue. The partner would get upset over Applejack's stubborn opinions, then leave in a rage. Every single time.
They'd always return next week, wanting to make up and get intimate again.
Applejack didn't understand her relationships at all.
But life seemed to work out, so she didn't question it much.
She was pleasantly surprised to see Barack Obama waiting in her bedchamber today. He had returned from his human world after several years of absence.
"My two terms are done. I don't have to lead a country anymore. Now I can spend all my time here with you, my beloved princess. I can offer you the love and happiness you deserve, whenever you want."
Princess Celestia panicked. She wasn't ready for a full-time, short-distance relationship.
"Um, are you looking for a place to stay? I believe there's some homes available in Ponyville," said Princess Celestia. "It's close enough that I can fly over and visit you when I get some free time. What do you say?"
She was horny because injured patients turned her on.
Ponyville was a safe town, and there usually weren't any patients available for her at the hospital.
But today Rainbow Dash was in traction after getting all four of her legs broken during angry sex with Fluttershy.
"Oh my," said Nurse Redheart. "You seem to end up in the hospital a lot. It must be awfully lonely, lying in bed while your body heals. Maybe I can... keep you company for the night?"
"OK, this is ridiculous," said Rainbow Dash. "How come I never get my own chapter in these things? Why do I always have to be the one who gets abused?"
"Oh, poor thing," said Nurse Redheart. "I didn't realize you were in such pain. Maybe I can help speed up your body's healing... by giving it a little TLC. I've always been interested in.... alternative medicine."
Rainbow Dash groaned. "I'm even in the cover art! That's totally misleading! Because I hate this story!"
Nurse Redheart anesthetized Rainbow's tongue so she would stop talking.
Rainbow's muffled protests made Nurse Redheart more horny than ever.