Julia is the lead character in these chapters. She takes the initiative in taking her illicit relationship with Winston to the next stage. She is the one who leaves him the little note which says "I love you"; she is the one who takes Winston to the secret hideaway where she's had sex with Party members before him. At every stage in Winston and Julia's relationship, it is Julia who takes the lead role.

In Oceania, such a forthright display of sexuality is strictly forbidden, especially for women, who are restricted to a subordinate role in society. Winston understands this and draws a sharp distinction between Julia's behavior and that of his first wife, Katherine. Unlike Julia, Katherine was always somewhat uncomfortable with physical intimacy. However, that's precisely what the Party would've wanted and expected from her. Julia, on the other hand, is an individual in the truest sense of the word, and that means defying the many constraints put upon her by both the Party and society, with their sexist assumptions of how a woman should behave.

If you are talking about traditional stereotypes from our own society (rather than anything within the 1984 society), they are reversed in the way that Winston and Julia behave.  Specifically, she is the aggressor and he is the one being pursued.

In our society, it is stereotypically the man's role to pursue a woman that he is interested in.  But in the chapters you mention, it is Julia who is pursuing.  It is Julia who puts the note in Winston's hand.  It is Julia who specifies when and where they will get together.  It is Julia who has had affairs before and it is Julia who is in control of their encounter.

So, this all goes against our sex roles -- the ones that say women should be more passive in romantic/sexual matters.