That's a fuckin' terrible joke -- or it's a good joke, not yet finished! Let's continue:

  Upon being asked, the prostitute laughs in their faces. "Either you two don't know how to fuck, or I don't know how to shit!"

This reminded me of another one:

    What’s the difference between a corn farmer with Tourette’s and a hooker with dysentery?

    One shucks between fits.

Instant classic. The ancients would’ve loved it.

Love it. Even modern Roman guys would be afraid of that ending ;)

That IS a good joke.

what was the birth control situation?

gold

Is it me or is that punchline not even crass? The wording the article uses to allude to it is equally crass, as far as I'm concerned (so what was the point of not including it?).

Agreed, I don't know what 'it involves asking a sex worker' spares us over that. Nor why we want to be spared in an article on 'jokes that have made people laugh for thousands of years' even if it were worse!

Either I am not getting the punchline behind the punchline, the prostitute is not very successful, or she has serious bowel problems.

It's a typical joke popular with the Carthaginians that the Romans all suffered from chronic constipation.

Is that so? That makes Incontinentya Buttocks even funnier.

that would be the opposite

hmm, yeah, it's hard to keep these old Carthaginian jokes straight here.

You had to be there, I guess.

You might be overestimating how often prostitutes have sex.

if i were a roman prostitute i'd have my clients use the back door to avoid getting pregnant.

Ah makes sense, that indeed makes it less unfunny

Or is being paid for...