I imagine there must've been an angle of "men away at war have needs" for many women like Calpurnia

I mean, orgies predate Rome.

Concubinage in Rome was actually not recognized except for unmarried folk, and then concubinage was still a (legally) monogamous contract that did not acknowledge one man with multiple concubines, while a man sleeping with his male friends would have been frowned upon - because a friendship is founded between equals, while Romans saw sexual penetration as inherently involving the domination of one party over the other. Sleep with actors and male prostitutes, like a decent citizen!

Open relationship hours!

Men were afforded much more leeway in adultery, but during the Late Republic, adultery amongst married men and married women alike was rampant. It appears to have been, at least partially, a matter of discretion and respect - a woman who carried on an affair or even several affairs could maintain both marriage and social position, so long as the affair was not an offense to her husband's dignity (ie by carrying on with someone filthy and low, like a BARBARIAN or a POOR) or too in-the-public-eye. There were not strong religious norms of sexual chastity in Roman society, which helped considerably.

That being said, Caesar did divorce his second wife over accusations of adultery, saying "Caesar's wife must be above suspicion." This was literal months after the 'suspicious' incident, though, and only after it turned into a massive political scandal, so there is certainly the urge to see it as a cynical move rather than genuine offense. That and Caesar was not close to his second wife, unlike his first wife (whom he defied a dictator's order to divorce, almost getting himself killed in the process) or his third wife.