Any student of Latin lyric poetry will tell you that Catullus' poems get pretty raunchy, obsessed with genitalia, semen, and sex in general. But one of his poems is so vulgar that an uncensored modern English translation wasn't published until the 20th century.
The text below includes a translation of the poem which is NSFW and includes sexually violent language.
Catullus' Carmen 16, sometimes referred to by its first line, "Paedicabo ego vos et irrumabo," is only of several poems addressed to two men: Marcus Furius Bibaculus (who had an affair with Catullus' young male lover Juventius) and Marcus Aurelius Cotta Maximus Messalinus. In most of his poems addressed to Furius and Aurelius, Catullus heaps abuse onto his cohorts, and in this particular one, he threatens them with explicit rape:
Paedicabo ego vos et irrumabo,
Aureli pathice et cinaede Furi,
qui me ex versiculis meis putastis,
quod sunt molliculi, parum pudicum.
Nam castum esse decet pium poetam
ipsum, versiculos nihil necesse est,
qui tum denique habent salem ac leporem,
si sunt molliculi ac parum pudici
et quod pruriat
non dico pueris, sed his pilosis,
qui duros nequeunt movere lumbos.
Vos quod milia multa basiorum
legistis, male me marem putatis?
Paedicabo ego vos et irrumabo.
From Wikipedia, here's a rather NSFW translation:
I will sodomize you and face-fuck you,
bottom Aurelius and catamite Furius,
you who think, because my poems
are sensitive, that I have no shame.
For it's proper for a devoted poet to be moral
himself, [but] in no way is it necessary for his poems.
In point of fact, these have wit and charm,
if they are sensitive and a little shameless,
and can arouse an itch,
and I don't mean in boys, but in those hairy old men
who can't get it up.
Because you've read my countless kisses,
you think less of me as a man?
I will sodomize you and face-fuck you.
And here's a more playful version published by Carl Sesar in 1974:
Up your ass and in your mouth
Aurelius, you too, Furius, you cocksuckers,
calling me dirt because my poems
have naughty naughty words in them.
Just the poet's got to be a boy scout
fellas, not his goddamn poems.
Anyway look, they've got wit, sass,
and sure they're lewd and lascivious,
and can get somebody pretty hard-up too,
I mean not just young kids, but you hairy guys
who can barely get your stiff asses going,
so just because you read about a lot of kisses
you want to put something nasty on me as a man?
Fuck you, up your ass and in your mouth.
Now certainly English-speaking people who could read Latin knew what was going on in the poem, and people would certainly translate it privately. (And it was translated into other languages.) But when the poem was translated—or perhaps more accurately adapted—into English for publication, the translators avoided literally translating certain parts of the poem. In his 1973 essay on the poem, University of Nebraska-Lincoln classics professor Thomas Nelson Winter notes, "Until recently, English as forthright as the Latin could never be printed."
Often the lines that were considered the most offensive—the first two lines and the repeated final line—were left out of English translation or replaced with the original Latin. Even many mid-20th century translations of the poem were extraordinarily coy with the first two lines; F. A. Wright's translation began, "I'll show you I'm a man," while Jack Lindsay's started with "Aurelius down, you'll knuckle under!/ Furius up! Admit your blunder!"
So why is this particular poem so filthy? Well, in the grand scheme of Latin poetry (or even Catullus), it's not that bad. But the reason that Catullus used this particular language, language of sexual penetration, seems to be due to some challenge to Catullus' masculinity. But Winter suggests that at the time it was written, Catullus' language wouldn't have been nearly as shocking as it was to later, English-speaking readers:
In the sense that this is the normal language of those to whom he directs the poem, it is not obscene. Obscenity, like beauty, is in the eyes of the beholder.
The implication is that it's not the writer, but the readers, who made the poem so vulgar.
"Catullus Purified: A Brief History of Carmen 16" [University of Nebraska-Lincoln via algrenion]