Sex positions are positions that two people can have sexual intercourse in. Many of them are named, although only one generally accepted position is named after a number: the Cueball and Megan try to approximate the shapes of the numbers, which they are given by the narrator, as sex positions.
They start with the classic 69, then represent the number 99 as "spooning" while standing and the number 71 as "doggy-style" sex over a table. They are then given ever more difficult numbers to attempt, first 34 maybe referencing rule 34 of the internet - Cueball exclaims Uh as he has no idea how to make a 3 maybe he's supposed to curl up in a fetal position , although Megan does try in vain to form a 4.
The narrator is so frustrated, as we are told in the title text, because he did not even get to ask them to do a continued fraction which likely would have involved many other people as the ultimate challenge.
An additional line in xkcd: volume 0 suggests posing to form the definition of the gamma function. The joke is just that Cueball is trying more different numbers by some mathematical calculations but Megan is visibly annoyed. At the title text Cueball is disappointed because they still have to work on more complicated numbers. I'm sure this joke is just mixing Sex and Math, not more. Standard rules of comic chat-bubbles tend to apply to most of Randels comics. Words spoken by characted are referenced by a line extending from the head, thoughts are contained inside of bubbles, and narrator speech is contained "outside" of the panels inside it's own box.
I don't know if I was the only one anticipating the possibility of Imaginary sex. Actually, there's arguably plenty of that already, possibly complementing all the Real sex that actually happens. My take? The whole thing is being narrated. The numbers are the voice over, the characters hear the narrator, and have to act out the numbers. They have trouble with 34 rule 34 , the square root of 8 could be anal, and the last one? Read it out loud, in 2 pie.
