We hear references to masturbation, oral fixation and a "castration complex."
Freud famously made sex the center of his psychoanalysis. Sex is indeed an important topic here, and the debaters concentrate especially on homosexuality. While Freud argues that there's nothing wrong with homosexual relationships, and that sex should not be hindered, Lewis argues otherwise.
Throughout the film, it's fairly obvious that his own daughter is in a lesbian relationship-one that Freud refuses to acknowledge. And he uncomfortably acknowledges to Lewis that, in his own field of psychoanalysis, lesbian tendencies are inherently related to the woman's relationship with her father.
Freud himself seems to have an unhealthy attachment to Anna. When a male suitor comes to discuss courting his daughter, Freud rejects the matter, saying that Anna's much too young to "experience any sexual feelings." (By 1939, Anna is in her early 40s.) He insists that Anna's entire being be wrapped up in his well-being. Anna accepts this, for the most part, much to the annoyance of her apparent lover, Dorothy Burlingham. (We see the two hold hands at one point.)
When Anna tries to tell her father of her own erotically themed dreams, he tells her to stop-wanting to ignore their subtext. But he, too, dreams of Anna. We see a snippet of that dream or hallucination, where Anna and an androgynous other are both statues embracing in an intimate manner, with one covering Anna's breast with his/her hand.