Hiya,
I know just enough about Roman history to appreciate how much they got right about the spirit of the time. They made a lot of creative decisions sure. They changed a lot of characters to make them more salacious, like Atia who iirc was known for being pretty dignified irl. They used Octavia to tell a bit of the story of Octavian's grand daughter and the sexual revolution of her time. They made a lot of changes. But generally speaking I think they got what made the Romans tick.
The one thing that bugged me tho was Vorenus in Gaul. He has a wife back in Rome and so refuses to touch another woman. My understanding was that the Gallic campaign was especially brutal, with one third butchered and one third enslaved. Added to that was Rome's misogyny and the sadism of their troops, I just find it really, really hard to believe a Roman soldier would typically be so restrained sexually when at war.
How believable was this? Was it common for roman soldiers to abstain from abusing their conquered victims sexually?
How would other Roman soldiers act if someone behaved that way? Would it be seen as a sign of weakness or as a sign of virtue?
I might be able to give an answer although from a different perspective than what you're asking. Also, there is a ton of show spoilers here so hopefully you've already finished it.
You'll remember there are times where Lucius Vorenus is referred to as a stoic or being stoic. There is a modern sense of the word that means, "a person who can endure pain or hardship without showing their feelings or complaining" (Oxford). But there is also Stoicism with a capital S that refers to the philosophy.
A couple things before I go further, it's clear that the show writers did not have a deep understanding of Stoicism, thinking of it only in these modern terms. If not the writers then certainly the characters did not fully understand which actually may fit into the world building. Although I don't think they call him a Stoic in the show, Cato is their best representation of one and he is also an educated senator so it would make sense that he had a good grasp of it. Marc Antony refers to Vorenus as "one of those Stoic types". An attempt was made to educate Antony but most accounts are that he spent most of his teens gambling and drinking (Britannica refers to his "dissipated youth"; UXL Encyclopedia of World Biography elaborates a little more with the same conclusion), but whatever education may have stuck his lifestyle certainly was not one of a Stoic. Meanwhile Lucius Vorenus was a common citizen and likely did not receive a formal education, so his understanding of Stoicism may have been incomplete or only adopted the parts that would be relevant to the life of a career soldier.
The show Rome takes place before the most prolific Roman Stoic writers were around. Seneca the Younger was born 4 BC. Epictetus (born 50 AD) and Marcus Aurelius (born 121 AD) came after everyone depicted on the show was likely already dead. For reference, the Battle of Actium where Octavian defeated Antony and Cleopatra was 31 BC. Stoicism got its start though with Zeno of Citium in Athens in 301 BC.
The primary virtues of Stoicism are: Wisdom/Prudence, Justice/Lawfulness/Integrity, Courage/Fortitude, Moderation/Temperance. The second and third group - justice/lawfulness/integrity and courage/fortitude - Vorenus certainly exemplifies and since that's not what your question is about I'll move on. The first one - wisdom/prudence - is more questionable. Donaldson defines it as, "excellent deliberation, good calculation, quick-wittedness, good sense, a healthy sense of purpose and resourcefulness". I think Vorenus demonstrates a lot of these qualities, but also does a lot of bone-headed stuff. However, this virtue is also not relevant to your question.
The last part is what is relevant to your question - Moderation/Temperance. Donaldson defines this as, "[opposing] intemperance or excess. Includes organization, orderliness, modesty, and self-control." That's a little vague but you can start to see the relevance. Vorenus was hot-headed to say the least and over indulges in drink on multiple occasions. This is also another point where Antony blunders about Stoicism. In that quote from him above he specifically says (and I'm recalling this the best I can from memory), "You stoic types often turn to drink when disappointed in life." That's straight up antithetical to Stoicism and belongs more with that modern interpretation of being stoic with a lower-case s, as someone who just bottles up their emotions.
Hand-in-hand with avoiding excessive drinking for a Stoic is avoiding unchecked lust and the pursuit of money. These all go together because it's about self-control and not being dependent on anything except your own faculties. To a Stoic, sex and money are not inherently good or bad, but over indulgence in either is because you're giving in to your passions. In both these things Vorenus shows up well as a Stoic. When Pollo shows up with the city's treasury, Vorenus isn't even tempted. He insists Pollo return it to Caesar ASAP. He also shows his discomfort for flashy clothes when his wife is dressing him after he gets dragged into politics.
So to your point, his aversion to extra-marital sex fits in with the overall character of Vorenus, the Stoic, instead of Vorenus, the soldier. While married to Niobe, he doesn't take any other women. Even when he talks to Pollo about sex he's asking for advice on how to make her happy which is a very matter-of-fact and sensible way to think about it. Further, even after she dies, he is through his mourning period, and he eventually does have sex with another woman years later it is with a prostitute who he promptly pays and sends on her way. There is no emotional tie to. It is clear he is just getting the desire out of the way so he can move on with the rest of his day.
To conclude, you're looking at it from the perspective of Vorenus as just a soldier. However, the show also demonstrates that he attempts to be a Stoic, even though there are short-comings on either the writers' part or character flaws with Vorenus. To a Stoic, indulging in sexual passions is a vice that needs to be kept in check and is one of the ways that Vorenus was a (mostly) good Stoic.
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