Marcus Aurelius was an Emperor, which means that he would have had a slew of what were essentially concubines. This would have been expected, and was so unremarkable that nobody would have even thought to comment on it. His enemies would have commented on his wife cheating because it undermines the idea that he is the authority of his house, him sleeping around would have affirmed it.

Even the most wife-guy emperors had mistresses. It was only commented on when the emperor was particularly disliked by the senate historians, they supposedly got particularly weird with it or everything was falling apart while they were around.