There might be several reasons for all the anal sex in Episode XV of Rome.

It might be as simple as they did a lot of consensual hetero sex in previous episodes, they worked in a consensual lesbian angle, so now its time for a little ruru. I suppose next week we get forced hetero sex.

Romans had an odd analysis of anal sex in that it was fine and dandy to be a giver, but it was base and disgusting to be a receiver. (Which is why most political campaigns seem to have started out by maligning the opposing candidate by claiming he played the woman's part, i.e., calling Caesar "every man's woman.")

It may be that this episode of Rome is editorializing about "power" in Rome. Who had it and how did they wield it?

Servilia has "power" in that she has wealth and social position. But a hired assassin thug has the power to coerce her into kissing him.

Atia has "power" in that she too has wealth and social position. But she prostitutes herself to her horse chandler and to Antony to get them to do what she wants.

The slave assassin has the power to kill Atia… but he is subservient to the major domo who demands anal sex to repay him for taking him off the street. And so the slave assassin coerces Servilia to kiss him…

It's all very symbolic if you want to look at it that way.