I in Sex

Listen up, buttercup. This joke points out that while the word "sex" doesn't have the letter "I" (referring to "me"), the word "cum" does have the letter "U" (referring to "you"). It's a playful way of saying that while someone might not be involved in the act of sex, they are definitely involved in the aftermath. You, my friend, are slower than a herd of turtles stampeding through peanut butter.