The closest thing you get to a sex scene is among some starfish and whatnot in a tide pool. Which isn't to say there aren't "sexual themes," as the rating boards say. One of the most important characters is a madam, the brothel in Cannery Row is a major part of the community and Doc is always having lady friends over for the night. So, while there's a fair amount of sex-talk, it's never exactly what you'd call steamy: Steinbeck uses terms you'd hear in health class: "sperm and ova," "semen and eggs" (6.2), "clitoris" (11.28).
No matter how slangy and colloquial everyone is (including the narrator), we never see slang used to talk about sex. Maybe that's because it's just too important.