I know that lots of cultures throughout time built penis statues and whatnot but they were being "serious"

Are you so sure about that? Sure there are things like Priapus which had religious significance (the second picture I linked is actually of a ritual windchime that seems to have been common in warding off the evil eye, not unlike Priapus) but ancient peoples had no issues with finding human genitals funny. Penises in particular seem to have gotten a kick out of both the Greeks and the Romans. We find graffiti at Pompei cracking jokes about sticking your weiner in a fire and the Colosseum has penises scribbled on it. For every Priapus or good luck charm with a penis there's a graffito like the one at the Domus Tiberiana with a guy whose nose is a giant penis (sadly I couldn't find a picture of it easily available). And Greek and Latin poetry abounds with jokes about penises--the Romans in particular loved coming up with different amusing euphemisms for the male genitalia. Hell, the Lysistrata is basically entirely composed of penis and vagina jokes. Old Comedy brings up a good point, that we don't have to stick depictions of penises into either "ritual" or "comic" roles. In Old Comedy the actors wore phalluses (note the penises dangling between the legs of the actors in [this[(http://www.usu.edu/markdamen/ClasDram/images/08/slavespusholdmanupladder.jpg) vase painting, which depicts a scene from an Old Comedy where two slaves are pushing an old guy up a ladder or ramp or something) that were technically ritual in nature--the cult of Dionysus was notably phallic in nature, featuring important phallic imagery and phallic processions. But it's nonsense to say that the Athenians didn't find those phalluses incredibly funny. Actors in Old Comedy often wore phalluses that were comically large or even tied up in knots, and were often...played with during performance. For example, the Lysistrata is full of jokes about erections, including a Lacedaemonian envoy comically burdened down by his "load" and unable to walk properly . The lack of a phallus was also extremely amusing. When the tragic poet Agathon, noted for his effeminacy, appeared as a character in the Thesmophoriazusae Aristophanes dressed his as a woman and appears not to have given the actor playing him a phallus, prompting the joke from Mnesilochus:

τίς δ᾽ αὐτὸς ὦ παῖ; πότερον ὡς ἀνὴρ τρέφει; καὶ ποῦ πέος; ποῦ χλαῖνα; ποῦ Λακωνικαί; ἀλλ᾽ ὡς γυνὴ δῆτ᾽: εἶτα ποῦ τὰ τιτθία;

But what are you supposed to be, child? Which do you pretend to be, a man? Where is your dick? Where is your cloak? [As opposed to the effeminate garments which Agathon is wearing] Where are your Laconian sandals? [Another type of distinctively male garment] But of course you must think you're a woman. Then where are your tits? [Some more prudish translators have rendered τιτθία as "breasts." They don't mean "breasts." They mean "tits," τιτθία is the diminutive of τιτθός, "nipple," a cognate to English "teat" and "tit"]

And jokes about genitalia were hardly confined to the penis. One of Aristophanes' most famous jokes is the pun that the Megaran makes in the Acharnians as he's about to sell his daughters disguised as piglets--piglet, χοῖρος, is a pun as the word was also a euphemism for the vagina, not unlike English "pussy." Aristophanes has lots of jokes like these--another is the Sausage-Seller's boast that a politician spotted him stealing meat as a boy, and proclaimed that the boy was bound for public office after the Sausage-Seller stuffed the meat between his buttocks and insisted he had done nothing wrong. An obvious joke about buggery and the passive role in male homosexual relationships, as well as one about the nature of Athenian politicians. There are graffiti at Pompeii that joke about penises abandoning women and only going after men's buttocks, or insulting guys for eating out women in back alleys, hairy vagina jokes, all kinds of stuff. People have found genitalia and sexuality funny literally since we've been people