It depends on what you mean by "sexually liberal." Roman society included and practically condoned systematic rape of slaves (both male and female), for example. While certainly promiscuous, I doubt any sane person would describe that as sexually liberal.

The Romans could also be quite strict on certain sexual taboos, most notably the taboo against a married woman having an affair. (Married men were given far more leeway, unless one slept with the wrong powerful man's wife)

The Emperor Augustus, in an attempt to get birth rates up in the aftermath of the civil wars, notoriously introduced some very strict laws condemning sex outside of marriage and promoting a return to monogamy, and was humiliated when it turned out that first his daughter, and then his granddaughter flagrantly violated these rules with many, many affairs.