One thing that irks me about the dialogue, is that they try and use ancient prose and terms, yet they use fcuk all over the place, it never sounds right when i hear it.
The etymology of the word is pretty much impossible to trace and therefore is a matter of (relatively unimportant) debate.
QUOTE
Via Latin or Greek
-Other possible connections are to Latin fūtuere (almost exactly the same meaning as the English verb "to ****"); but it would have to be explained how the word reached Scandinavia from Roman contact, and how the t became k.[citation needed] From fūtuere came French foutre, Catalan fotre, Italian fottere, Romanian futere, vulgar peninsular Spanish follar and joder, and Portuguese foder. However, there is considerable doubt and no clear lineage for these derivations. These roots, even if cognates, are not the original Indo-European word for to copulate, but Wayland Young (who agrees that these words are related) argues that they derive from the Indo-European *bhu– or *bhug– ("be", "become"), or as causative "create" [see Young, 1964]. A possible intermediate might be a Latin 4th-declension verbal noun *fūtus, with possible meanings including "act of (pro)creating".
-A derivation from Latin facere = "to do", "to make" has been suggested.[citation needed]
-Greek phyō (φυω) has various meanings, including (of a man) "to beget", or (of a woman), "to give birth to".[7] Its perfect tense pephyka (πεφυκα) can be likened to "****" and its equivalents in other Germanic languages
So, while the exact word itself wouldn't have been in Ancient Rome, some variation of it possibly would have been, so it's not completely out of order to have them use it...after all, they do have them speaking english, just in that era's style. They use very few, if any (I'm not sure), words and terms that aren't directly out of our vernacular.