In the HBO show rome, mark Anthony says to the "liberators" that after his turn รณ office he will "retire to the countryside, tend to my fields and fuck my slaves.". Was this common among the Roman population, and if so was it more common for a roman citizen ti fuck his slave rather than his wife or fiance?
As with many questions of this nature, it is difficult to answer 'Yes' or 'No' to "was this common?"
Let's look at it legally. The only thing that really governs sex in Roman law, stuprum is that it must not be adulterous, most types of rape, or a couple of other very specific cases. However, stuprum is not a protection slaves enjoy - only citizen on citizen cases are illegal. Slaves, especially female slaves, were definitely open to the 'use' of by their master, and were treated much like property.
Cicero writes in his letters about a male slave who he enjoyed in this fashion, and you can also see it mentioned in political attacks, especially in reference to either a mother or a wife, (I believe there is a circulation that Nero's wife, Octavia, has sex with her slaves, but I could not find that myself.) In any case, it is definitely a scandalous thing for a woman, even if she be the 'owner' of slaves (owner in quotes due to varying levels of ownership women can even claim), to have sex with them.
The only thing I can say as to your question concerning the frequency of it is that I saw quite a few of them in legal cases involving property disputes (ie, Marcus accuses neighbor, Caelius, of raping a serving girl of Marcus' household, and fathering a child), but really nothing to base a "Oh, it happened all the time!" generalization on.
Edit: just throwing in some sources and my own credentials, I studied Classics and Roman Civ, with my thesis being on Roman Law's interactions with neighboring areas. Cicero's letters do mention the male slave relationship (was it Tiro? I don't think it was...), but really no specific mention of anyone retiring to their villas and plowing anything except for their fields for olives and such...
Edit 2: I want to specify, undergrad thesis, not grad study. And I focused on non-European successors.
A comment posted on here a while back about the Catholic church's sexual restraint in that period mentioned that it did declare that it was wrong to have sex with one's own slaves, so it was most certainly going on among a portion of slaveowners.