Title text: Hey, I don't make the rules. It's in the book.
These two friends (both presumably male, since female characters in xkcd are depicted with hair) are surprisingly cavalier in taking the suggestion to engage in sexual experimentation to alleviate boredom. Even if both men are gay, the fact that they're friends (as the rule in the book describes them) suggests that they are not currently having sex on a regular basis. In this case - and even more so if the friends are heterosexual - most people would not take the book's suggestion and it may even make them feel embarrassed and awkward.
The book that one of the Cueballs grabs appears to be some sort of all-encompassing rule book, its reach including the social sphere. Obviously this book is fictional, but the line "I think there's a rule about this" sounds like a reference to folk "rules" or guidelines like the "five-second rule."
Crisco is a brand of shortening, a fat that is solid at room temperature and frequently used in baking. In this instance it's implied that it will be used as a sexual lubricant. Because Crisco is not commonly used as a sexual lubricant, their intended use of it hearkens back to the suggestion to engage in sexual experimentation rather than typical sexual activity.
The title text could be spoken by either one of the characters or Randall. It attempts to preempt any awkwardness or judgment the reader may have about this situation by tranfering responsibility to the rule book.
- Cueball: So what do you want to do?
- Friend: Still no ideas.
- Cueball: Wait, I think there's a rule about this.
- [Cueball goes to bookshelf and removes a book called "Rules".]
- [The book of rules is opened to the following:
- RULE social.b.99.1
- If friends spend more than 60 minutes deciding what to do, they must default to sexual experimentation.]
- [Cueball is standing, holding the book. The friend is in the process of standing up.]
- Cueball: Huh.
- Friend: I did not know that rule.
- Cueball: Me neither.
- Friend: I'll go get the Crisco.