How did that society come about
What was the culture like before then
What was sexuality like in that culture and how did it lead to the Roman sexualilty known for its openness and 'debauchery'
The overall premise of your question's flawed because Roman society was neither open to alternative and "free" sexuality nor was it egalitarian. It was extraordinarily repressive, conservative, patriarchial and rigidly defined in it's notions of sexual morality and sexuality. All of our literary sources and most of our evidence deals with the viewpoint of the aristocratic male elite which really screws up our attempts to paint a comprehensive picture of Roman society but it also reflects the hierarchy of Roman society. This elite also enjoyed the most liberty (sexual or otherwise) of any demographic in the Roman world but even they were bound by strong social mores and taboos.
You should check out these answers on the subject:
What was it like being gay in ancient Rome? How socially acceptable did the Romans view it? by /u/thebatteryhuman and /u/MarsTheGodOfWar
What would usually happen to a Roman man who slept with someone's wife? by /u/XenophontheAthenian
Is it true that women in the ancient Mediterranean wore veiled clothing similar to the way women in the Middle East do? by /u/chocolatepot
Wednesday AMA: I am heyheymse, specialist in Roman sexuality and mod of this fine community! AMA.
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