Huxley definitely has a unique style of narration, where he cuts back and forth between simulataneous converations that people are having, almost like the movies do now, where editing allows us to follow what his happening with multiple characters at the same time, using split screens. And this narration wasn't random or purposeless, it was a way for Huxley to make symbolic connections, and convey a lot of information in a little amount of time.
In the sequence between the controller and Fanny and Lenina, Huxley uses the Controller to describe the process by which people are genetically engineered to behave in a certain way; Fanny and Lenina are like the physical manifestation of that genetic engineering. It is like Huxley is giving us a real-life example of the results of what the Controller is talking about. He is giving a lecture, giving straight information to people in pretty scientific terms. But to better understand the impact of those scientific experiements, it helps to see an example of how they turn out in the end. Fanny and Lenina, discussing men, their sex lives, the social atmosphere, unwritten social rules and laws that reveal their society's mindset, and the different types of social classes that are created through the engineering, help us to see first-hand just exactly what the Controller is talking about. It is no longer an abstract concept that we have to imagine, Huxley paints the picture for us to help us understand how it works in their world. Lenina and Fanny are the physical manifestation of the Controller's lectures.
It's an interesting approach to story-telling, but does a good job of helping the reader to more fully process the information on more than one level. I hope that those thoughts helped; good luck!
I believe that it is put so close to this conversation to serve as a contrast. It is a contrast between the way that most people are -- the way the government wants them to be -- and the way that Lenina is behaving.
Lenina is something of a rebel. She have been going out with one guy exclusively for four months now instead of sleeping around. Later, she will be the subject of some amount of competition between various men. This kind of thing isn't supposed to happen.
By putting this conversation right next to the Controller's lecture on the subject of family and intimacy, Huxley emphasizes how subversive her behavior is.