She's been cuddling that new Judgeradicator Joodiy datachip against her chest all day, ever since Jade and Rose came back from their groceries expedition, so it's not too hard to figure out that when she sneaks away after dinner it's to go marathon it. She will be utterly engrossed for several hours, she's said so. Perfect time for a prank!
He bursts in the rec room with a bag of the most disgusting rotten-purple and snot-green confetti at the ready, and ends up frozen in the middle of the carpet, face to face with Terezi-bitten Dave asscheeks, jutting impudently.
"Oh my god," says John, and Dave shrieks "oh my god!", and Terezi stares at John (kinda) and lets out a triumphant "Bwahaha!" in the purest tradition of Disney villainy.
(Disney is now a trollish franchise. That makes it pretty villainous.)
The way she laughs at his retreat has his ears heat up even worse than the fact that he got an eyeful of his best bro's butt.
He cannot get it out of his mind, even as he starts up the cycle again with other pranks, other topics. This time she bested him, and that can't be allowed. He can't figure how to top that, though. She pretty much made him prank himself.
Also, Dave butt. Okay, no. Well, Dave butt is pretty tolerable when it's doing things to Jade butt or (maybe in theory yes) Karkat butt (might want to sneak a quick, discreet look some day) because he loves watching his lovers fall to pieces, but on its own it makes John want to claw out his eyes.
It was pretty nice how quick Dave was to help him make Jade make pretty noises that day but that is so totally off topic, wow, he is totally not thinking of that at odd times pretty much ever since, no.
But anyway. Terezi. The fiend from hell. She used Dave against him! His best bro!
It's totally fair that he use Dave against her.
... Oh. Huh.
The occasion comes before he's done wrestling the thought into submission, so for the longest time he just -- he watches, and he doubts, and he tells himself it's not the right time and Dave wouldn't go along with it anyway and then it'd flop miserably, and --
And Jade knows he's here in the doorway they left open because she's facing it but she has Terezi's tongue up her umm womanplace so it's pretty likely Terezi can't scent him, she's got to be smelling nothing but Jade's happy juices.
Dave is on his knees behind her, petting her butt.
John has to admit Terezi has a nice butt. Nicer than Jade's even. (Jade has better boobs though and also the rest of her is not made of pointy bits and she would never yell surprise buttsex and flip him over, which he's sure Terezi would and in other news he is never going to do anything with her directly. Ever.)
(Then again he's never going to do anything with Dave directly either, but...)
He moves silently on the carpet, and Jade cracks her eyes open and smiles at him (he's sure they left the door open as a standing invitation to join in, why else, so it's okay). He breathes in, and then he puts his hands on Dave's bare shoulders (it's okay! John's dressed! John's in jeans and t-shirt and staying in them!)
Dave startles, begins to turn, one of his hands on Terezi's hip, the other resting on the curve of her butt, and John leans in to whisper in his ear, "Slap it."
(He's not sure why it comes out all rough and low and -- and like that, but (Dave shudders all over (he is not noticing that)) Dave's hand comes down, almost on automatic, swift and without hesitation.
The way Terezi squeaks and jumps out of Jade's crotch has him laughing so hard he almost falls over. She turns, bewildered, then glares -- oh, sweet victory.
"Yee-haw," Dave mumbles, blinking down at his hand. His face has gone all red around the shades. Jade is laughing her butt off.
And Terezi is growling! Whoops, time to abscond.
He does, right into Karkat's waiting chest.
Considering the amount of naked in the room it's pretty weird how displeased Karkat looks. John tries on a friendly grin. Behind him, Terezi pinches his hip, hard, and grins in turn. "Mister Cherry Delight! How nice to--"
"The two of you will stop pitch-flirting with Dave as a proxy immediately, or else me and Jade are taking him back and going home. I am not letting the two of you make that into a thing."
His eyes are all squinty. Dave opens his mouth like he's about to say he agrees, or maybe that he doesn't mind it becoming a thing (John can see it either way.)
"Yeah, seriously," Jade pipes up before he can speak, "If you're gonna use an intermediary to get your blackflirting out of your system, it's kind of cheating on him not to use Karkat! He is your auspistice after all!"
"... That is so not what I was saying," Karkat sputters, but, well.
It's super disturbing to hear Terezi's happy noises up close and personal. She sounds like a chainsaw in love, nothing like Karkat's motor-trying-to-start-with-a-kitten-on-it.
At the same time it's hella fun to imagine the grimaces she must make when he makes Karkat gasp or moan before she can.
They keep making love to him in the dark, and if any hands stray, Karkat is there to distract John before he shrieks and put them back where they should be.
(It occurs to him later that Terezi is blind anyway and therefore that she totally did see him naked.)