The things you know about ancient Rome from the movies involve the elite, the movers and shakers, and certainly didn't apply to everyone. And those movies also tend to incorporate modern values and customs, so they don't necessarily reflect the accepted values of the time. Sex in ancient Rome was unconstrained by the monotheistic religions, which either didn't exist yet or were relatively obscure to the Romans. But there were rules, mainly in place to maintain social status, because status was everything. Those with social status followed those rules, unless you were royalty, for which all the rules went out the window. After all, what can indicate status better than being above the rules? If you were poor, working class, from a conquered nation or ethnicity, a woman, or a slave, you were sure to get the short end of the stick, so to speak. Weird History explains the rules for sex in ancient Rome and who broke them the most.