So that means a male slave or plebeian could perform oral sex on a noblewoman?
Yes.
I might add that this was a public taboo. It's likely that people were still practicing the very things that were forbidden, in private, because there was always rumor about it.
Btw, as far as adultery goes, it only applied to women. If a man had sex with someone other than his wife, that was ok, but if a woman had sex with someone other than her husband, that was not ok. The reason is because women were treated as property, whose value is reproductive. If your wife had sex with another man, there was no way to know if her child was yours. Again, it's clear that Roman wives were sleeping around, because we hear so much about it.
In ancient Rome, regardless of your gender or social status, sex was widespread and easy enough to obtain. Even if you were not very appealing, you could solicit a prostitute (male or female), which was perfectly legal. If you owned any slaves, you could compel them to have sex with you. There was no notion that sex had anything to do with love, so people tended to do it with less hesitation. Sex was considered a biological urge, no different than hunger, sleep, or thirst. In fact, a husband who actually loved his wife was said to be naive.
Most of these practices began to disappear through the influence of philosophy and Christianity. Christianity, in particular, caused sex to be viewed as a necessary evil, but one which the pious should indulge in as infrequently as possible. As the influence of religion has decreased in the last 500 years, so has its influence over sexuality, and we are seeing a gradual return to the ancient Roman notion of sex being purely a leisure activity, rather than an act of love.