1. So much (casual) sex: There's a lot of sex in the comedy -- discussion of it, particularly the casual hook-up culture in which young men and women meet each other at a bar, and without even a name to go on, end up going home with each other for a one-night stand. If it's not a one-night stand, the sex is of a no-strings-attached nature which makes booty calls all the easier for the characters Jason (Efron), Daniel (Teller), and Mikey (Jordan).
2. The sex shop: There's an extensive sequence in the movie when two of the guys go to an adult-novelty shop and check out the wares -- most of them vibrators, penis pumps, and sex toys of all sorts. Before settling on a strap-on, the guys inspect the store's adult products, make innuendo-filled jokes about size and performance and why they would need these novelties.
3. The strap-on costume: Jason goes to a dress-up party thinking it's a costume party, but the party is actually fancy-dress party not a costume one. Jason shows up with a bandana, muscle-shirt that says "Rock Out" ...and a huge fake penis hanging out of his pants in order to embody the idea of "Rock Out With Your C**k Out." The fake penis becomes a raunchy sight gag for the rest of the party, with him thrusting it while dancing and making jokes about how it compares to his own penis.
4. The orange penis joke: Early in the movie, Mikey, still understandably upset about his divorce, decides to masturbate instead of picking up a woman for a one-night stand. What he doesn't realize is that the lotion he thinks he's using is actually self tanner, so the next day he reveals his orange penis to his friends (it's not visible to the audience), thereby launching a seemingly endless series of jokes about what his penis looks like.
5. Viagra jokes: Early in the movie, the guys pop Viagra and then go out on the town. Later, after having sex, Jason and Daniel can't get their erections to subside and end up having to get completely horizontal to pee. The scene shows both Efron and Teller bottomless and trying to pee, but they can't really accomplish that without having their penises touch the toilet water. It's crude and scatological, but that's exactly what the movie's humor is based on.