Well it depends on your view of liberated. Women were about as un-liberated as you can get.* Female slaves (and male, really slaves of any shape or size) had no bodily autonomy whatsoever (ie. they could raped at any time by their owner, though not by somebody else as that was basically damaging another person's property). Women, children and less often men were raped as a matter of course in warfare between all cultures, and Rome being the victors more often the not they did the lion's share of such sexual violence (for more on this see Gaca). Married women were expected to be chaste, the key virtue for females being pudicitia (modesty, sexual virtue) as opposed to virtus (manliness, courage) for men. In practicality this meant that women had to be absolutely faithful to their husband, but the husband could sleep with slaves and prostitutes if he so desired without it being considered wrong.

Ancient Romans did not have a lot of "homosexual relations", indeed they had little concept of homosexuality as we understand it today. Sex being considered an act of manly domination in the ancient world, there was nothing remotely manly about being on the receiving end of any sort of man-on-man intercourse, as it was not particularly manly to play the submissive role with a woman. So giving oral sex was out for any man, as was receiving anal sex, and even being on the bottom with a woman could be considered to be 'passive' and therefore unmanly.

With citizens of good standing however, there were things that could very clearly be done and things that could be done. In fact, unlike in modern Western society, consensual sex in Ancient Rome could be criminal irregardless of consent depending on who was doing it with whom. Stuprum was a sexual crime, and could cover sex with a married woman as well as sex with a citizen youth (Skinner 258). Unlike Greece, pederasty between citizens did not occur in Rome, it would have been considered a disgusting and criminal act.

Criminal sex could have major ramifications. Sex with a married woman was out. Even flirting with a citizen woman or youth in public could be criminal (261). The Romans took an extremely dim view of sexualised behaviour in public of any description: indeed, husbands and their wives generally did not interact in public to my knowledge. Adultery was in the Augustan period (Augustus was a real stickler for sexual ethics: he may have had his daughter killed for immorality) a crime against the state and the gods, though in Rome it was usually the woman that was to blame (258). On the other hand, sex with a vestal virgin was one of the most extreme crimes one could commit and not only got the vestal buried alive, but the male partner if caught would be publicly tortured to death (perhaps accidentally) before being beheaded, the most shameful of deaths and normally illegal for a citizen.

Sexual morality, especially as regards married women, was viewed as vital to the good health of the res publica and the discipline of the state (266-7). While I said above women tended to be blamed for public scandals, under Augustus laws were passed that did make men criminally liable also. A father finding his daughter in bed with a man outside of wedlock or with a man other than her husband was protected by the law if he chose to slaughter them both on the spot (266-7). A husband could kill the lower-class lover of his wife (such as if they were an actor or a gladiator), though he could not kill his wife, but it was also illegal for him to know of an affair and take no action (268).

I'm far from an expert on Rome sexual mores, but the above should give you a bit of an overview and demonstrate that Roman sexuality was a lot more complicated that it seems in HBO's Rome. It might also be worth wondering if the 'doggy style' sexual position was as common as shown on TV among aristocrats, considering the Romans had a term for it, coitus more ferarum (sex in the manner of wild beasts), which doesn't exactly carry positive connotations. The Roman upper class were not sex-mad maniacs who had sex with everything and everyone they saw, but took sexual morality very seriously and operated under pretty strict social and legal restrictions regarding sex. Basically, if you were a married citizen man, it was pretty damned "sexually liberated", as you had a wife, you could have sex with slaves and prostitutes, and on campaign you could commit all the rape you like. But there were quite a number of things you categorically could not do, and for the victims of sexual violence (such as slaves and the populations of captured cities) and for women in general, I wouldn't say it was particularly "liberated" at all. Also remember that a man who had sex with the wrong person at the wrong time could find himself being scourged to death in the forum or taking a knife in the gut from an enraged father or husband: it wasn't exactly open season.

Essentially, when watching HBO's Rome I'd stopped taking it seriously well before this point but (spoilers!!!!) the part where Octavian starts sleeping with his sister (super illegal) was the part where I stopped wondering whether or not the writers and producers cared at all to provide an 'authentic' depiction Roman sexuality as opposed to what they gave us, which was just run-of-the-mill porn written by people who were too blinded by dollar-signs to do their homework. That said, I've watched HBO's Rome like 4 times so maybe they were onto something...

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*Sexually that is. Roman women might be considered fairly liberated for their time in terms of their financial and social independence, and were jealously protected by their families.

Sources:

Gaca, Kathy L. “Girls, Women and the Significance of Sexual Violence in Ancient Warfare,” from Sexual Violence in Conflict Zones: From the Ancient World to the Era of Human Rights. Ed. Elizabeth D. Heineman. Philadelphia: University of Pennsylvania Press, 2011, 73-88.

Skinner, Marilyn B. Sexuality in Greek and Roman Culture: Second Edition. Oxford: Blackwell Publishing Ltd., 2005.