They were baking a cake for Kenny's birthday.
It was one of Kyle's stupidly noble ideas, seeing how apparently Kenny was too poor to get a cake himself or something. Cartman couldn't be bothered to pay attention, because he knew that Stan would agree instantly, and besides, it's not like he was going to do anything anyway, except from being present when they were baking it and taking some of the credit afterwards.
The scene was something like this: They were in Kyle's kitchen. The aforementioned was wearing a pink lacy apron, with some Yiddish puny phrase printed in the front, and mixing a bowl of something chocolaty. Stan was sitting on the counter, chatting with him idly, while also mixing some sort of white creamy thing. Again, Cartman was too bothered to pay attention, even though there was food involved, but now for completely different reasons: he couldn't stop staring at Kyle.
No, really. He had tried to distract himself, first with some game on his phone, then bitching at his friends. He ended losing the game, and stuttering some really lame insults, which only made the other boys suspicious of him. Meaning he now had to play cool for a while, but that was impossible. He had just discovered a fetish he didn't even know he had, and a very specific one at that: Kyle in a pink apron, baking or cooking or anything that had him being feminine in a kitchen.
It wasn't the discovery that surprised him. Since puberty, his list of fetishes and fantasies was growing every day, and somehow they all involved Kyle doing something. It started with the basic ones, like fighting each other, or Kyle's hair, but suddenly started amounting to almost every interaction they had. Luckily for Cartman, this meant he had it under control, because they always did the same things and Kyle was really predictable.
What shocked him, muted him, was exactly the unpredictability of it all. He had never, in all his life, expected to see Kyle in a pink apron. He may have fantasized about maid outfits and nothing beneath them, but he never believed it would come true. In his mind, Kyle was too proud to lower himself to that (and that's what made it all so good, the humiliation).
But apparently he was wrong. Kyle didn't care or felt ashamed about wearing feminine stuff. In retrospect, it made terrible sense: of course he would be all anti-gender and politically correct. It was just a bad character read from Cartman's side to think Kyle would be worried about stuff like that.
"Cartman, are you all right? You haven't said anything horrible in half an hour." He came out of his trance to see Kyle looking at him, worried. Stan was nowhere in sight. He had to reroute the conversation.
"Where is the hippie?"
"Oh, Stan went out to buy some cherries. We were going to ask you to do it, since you didn't help at all, but you were too spaced out." He didn't think he could change the topic this time, or even answer, because Kyle was standing there smirking at him while drying his hands on the apron. It was all too much. "Cartman, seriously, you're starting to freak me out."
"Fuck. Is that your mom's apron?" The phrase came out of his mouth before he could stop himself, and Kyle's worried frown changed into an eye roll.
"I knew it was taking you too long to comment on that. Yes, Cartman, I'm wearing my mom's pink apron. Happy?"
Oh, yes, he was happy. He loved when the Jew got bitchy. And if Kyle was already, unknowingly, making him hard just by saying that, what he did next was the push that broke the dam of Cartman's control. He bent over to peer inside the oven, pointing his round little ass at Cartman, and said "The cake is looking good, I think."
He just couldn't help himself. Cartman crossed the room in a second, cornering a surprised Kyle against the oven. He had his arms around Kyle, one hand deep into his curly hair, and kissing him like it was his last chance (in his defense, he thought it was). They kissed like that, messy and wet, until Kyle put a hand on his chest and pushed him. He felt like his world was collapsing, every part of his body and mind shutting down, because Kyle was rejecting him, it had to be, he was grossed out and fuck, Cartman had always been horrible to him, it was true–
But Kyle was dragging him by his wrist and soon they were on the sofa. He must have looked terribly confused, because Kyle said "My ass was burning", and everything was all right again, the weight was off his shoulders, they could go back to kissing messily on the sofa.
An hour or a year later, Cartman really couldn't be be bothered to know, someone said "The cake burned, you dicks." They separated, and to his delight Kyle was flushing, to see Stan looking fondly at them, a badly burned cake on his hands.
'Sweet,', Cartman thought. 'I get to see the Jew in an apron again.'