Applejack has known for years, thanks to accidentally walking in on him while waking up a couple times, that Big Mac prefers to sleep in the nude. It’s just how he feels most comfortable. That said, she posed a rule to him that he always wear something when Apple Blooms friends are over, even when he’s sleeping. After so long of no pony coming into his room until he’s ready to greet the world, he felt there was nothing wrong with ignoring that rule for just one night.
 
 
So imagine his complete shock when his youngest sister and her friends blow through his bedroom door in the morning as he’s getting out of bed with a question to ask him.
 
 
Sweetie was taken home not much longer after the event; stricken silent in fear of the sight she had seen. She had to be consoled by Rarity every night for weeks due to nightmares of the giant, red crotch snake. (On that note, I like to think this is how :iconmustlovefrogs: version of Sweetie became a lesbian :D)
 
 
Scootaloo, on the other hand, doubled down on her sexuality. After seeing the godlike gift Big Mac had hidden under his jeans, she became excited, and couldn’t wait to ask her parents what it was all about. Only a couple weeks later, she could be seen spending a lot more time with Truffle Shuffle. They’ve been going out ever since.
 
 
Apple Bloom would never be the same again.
 
 
Or at least that’s what she said after the three of them left the room. For the next couple days she just couldn’t bring herself to look Big Mac in the eyes. In fact, she tried to avoid him as much as she possibly could. It was just too awkward. Once Applejack realized what was happening, she pulled Apple Bloom to the side and sat her down for a talk. After explaining exactly why it’s silly for her to avoid her brother because of an accident, things went back to normal on the farm.
 
Applejack didn’t let the incident bother her. This wouldn’t be the first time she’s seen him naked. She was just happy he wasn’t…doing anything else this time.
 
Big Mac continued to sleep in the nude. With the help of his brand new door lock, nothing like this has ever happened again.