Alright, let's try and be mature about this. Subject of hilarity or not, sex is a recurring element within the story, and so, what with our task being to describe recurring elements within the story, it seems logical to add it in. Please keep any sniggers to yourself, you immature child, you.
On page 57 there is a photograph of two tortoises mating. The author may be trying to make some sort of point about awkwardness and perseverance, as the tortoises don't look very comfortable but will probably keep at it anyway.
On page 84 the story is told from Oskar's grandmother's point of view. She describes how she would model for sculptures Oskar's grandfather would make, and how they eventually become more intimate and ended up making love.
Oskar asks his mother about her new "friend" whom he suspects is a little bit more than a friend:
"True or False: you are in love with Ron." She put her hand with the ring on it in her hair and said, "Oskar, Ron is my friend." I was going to ask her if she was humping her friend, and if she said yes, I would have run away, and if she said no, I would have asked if they heavy-petted each other, which I know about." (35)
This also ties in with things Oskar knows about but doesn't want to.
"...without any experience I knew what to do, it was exactly as it had been in my dreams, as if all the information had been coiled within me like a spring, everything that was happening had happened before and would happen again, 'I don't recognize the world anymore,' Anna's father said, Anna rolled onto her back, behind a wall of books through which voices and pipe smoke escaped, 'I want to make love,' Anna whispered, I knew exactly what to do, night was arriving, trains were departing, I lifted her skirt, Mr. Goldberg said, 'I've never recognized it more,' and I could hear him breathing on the other side of the books, if he had take one from the shelf he would see everything. But the books protected us. I was in her for only a second before I burst into flames, she whimpered, Mr. Goldberg stomped his foot and let out a cry like a wounded animal, I asked her if she was upset, she shook her head no..."
This passage is very interesting because Mr. Goldberg and Anna's father's conversation related to Anna and Thomas's intercourse because many of the things they say are related to thins that are happening to Thomas and Anna. Just as Thomas didn't know how to do, this new "world" was new to him, Anna's dad says he doesn't recognize the world anymore. Then, just as Thomas knows exactly what to do, Mr. Goldberg says he's never recognized it more. Then Anna whimpered as Mr. Goldberg cried out. This also relates heavily to Oskar's tone in the whole story - he lists different subjects without connecting them, and he does bring sex up pretty frequently.
This quotation from after the narrator and Anna make love is ironic because he sees his future wife while doing it with whom he'll later do it, so that's something different.
"I know a lot about birds and bees, but I don't know very much about the birds and the bees." (192)
"Marylin Monroe was lost in the index for more than a decade! I kept checking under Norma Jean Baker, thinking I was smart, but completely forgetting that she was born Normsa Jean Mortenson!" "Who's Norma Jean Mortenson?" "Marylin Monroe!" "Who's Marylin Monroe?" "Sex!"
"We made love for the last time, I was with her and with everyone, when she got up to go to the bathroom there was blood on the sheets..."(281)
I think this quote is important because it contains a lot of foreshadowing, saying that it was their last time they made love, and how there was blood on the sheets afterwards. This last bit is especially strange, as if it was the last time that they made love then it is implied that they had done it before. Blood being on the sheets is what normally happens during the first time, so it being there after the last time is strange.
"I know a lot about birds and bees, but I don't know very much about the birds and the bees." (192)
I think the author references the theme of sex frequently in the book because it creates a sense of awkwardness that is funny yet understandable. It gives Oskar character while also setting a certain tone for the book. I remember reading these passages and my jaw dropped at how open these kids were to conversation, let alone how much they knew about the subject.