Faustina the Younger

Perhaps it's rooted in fact, though sources are a bit uncertain. Many allege that the Empress was of a rather promiscuous nature, while others say that this was libelous salacity to be taken with a grain of salt. Actually, it seems that Faustina's love life was quite the subject of lively debate among the Roman historians of the past few centuries.

The most thoughtful examination I was able to find was in the book East and West Through Fifteen Centuries, which—without taking a stance—nicely sums up some of the contradictions:

Dion Cassius asserts that Faustina was in love with Avidius Cassius and encouraged him in this revolt. But, curiously enough, the anonymous writer of the 4th century rejects this statement, and says that the reports to that effect were merely due to "a wish to defame the empress." Moreover, a letter to Marcus Aurelius from Faustina (who had apparently been left behind in Rome when he proceeded to the Rhine) in regard to this rebellion urges him not to spare Avidius Cassius and his accomplices, saying:

"You see how young Commodus is, and our son-in-law Pompeianus is old and is away. Do not spare men who have not spared you, and would not spare me and the children if they won." [Context available here]

So, if Faustina and Cassius truly were having an affair, it's unlikely that the Empress would have expressed this sentiment. But it's unclear where exactly Dion Cassius wrote that because it doesn't seem to be present in any of his works.

(More details could maybe be found here, though snippet view cuts it off)

Let's look at some of the chatter to get a sense of the rumors flying around. (I exaggerated when I said it was 'lively,' some of it's a bit dreary.)

The Encyclopaedia Britannica of 1894 tells us to ignore the accounts of the Empress's infidelity:

Faustina, who had borne [Aurelius] eleven children, died. The gossiping historians of the time, particularly Dion Cassius and Capitolinus, charge Faustina with the most shameless infidelity to her husband, who is even blamed for not paying heed to her crimes. But none of these stories rest on evidence which can fairly be considered trustworthy; while, on the other hand, there can be no doubt whatever that Aurelius loved his wife tenderly, and trusted her implicitly while she lived, and mourned deeply for her loss.

One of those "gossiping historians" is Tenney Frank, who writes in his A History of Rome:

On his return journey, the emperor's wife, Faustina, to whom he was deeply devoted despite rumors of her infidelity, died.

And a more specific claim is made in Historia Augusta:

It is generally known that Faustina, while at Caieta, used to choose out lovers from among the sailors and gladiators.

The book claims that some of Faustina's lovers included Tertullus, Tutilius​, Orfitus, and Moderatus. (Whoever they are.)

So: Was the Empress having an affair with Avidius Cassius?

Answer: Maybe.