Winston has had limited and negative experiences with "party women," or, women who are completely loyal subject to the ideologies and preachings of party values. His wife was one of them; she was a cold and unfeeling woman who filtered every emotion through the Party's expectations of her. Intimacy was merely a duty performed to produce children. Marriage was the expected ritual to undertake in order to further society. The only true feeling she showed was fervor for Party ideals. This left Winston alienated and lonely, and eventually, they split up, because there was no true foundation of affection or closeness between them.
With that experience as a foundation, Winston fit most other women he sees into that category. He sees Julia with her slogans and perfect attitude at work, and can't help but fit her into the mindless, robotic slaves of the Party that he has found most women to be. However, when he meets her, he realizes that she is in fact quite the opposite. She has her own beliefs, hates the Party, and uses its rituals and practices as a front for rebellion whenever she can. Winston is completely surprised at this. He didn't expect it, especially from her, who he always saw participating in the right activities in the right way. She helps him to realize that not all women believe the act, even if they are playing the part. This discovery is like a miraculous revelation to him, and changes his entire world.
I hope that helped; good luck!
Winston seems to have the view that women would be like his wife: ready to fulfill the duties of a wife as defined by the Party. This means sex is only for procreative purposes and is not pleasurable or fulfilling in any way. Julia is a completely opposite woman.
Julia carefully plans their first encounter as to accomodate sex if they wish, and for the purpose of pleasure. As their relationship grows, she tends to introduce opportunity and loves him fully with her whole being. All Winston had ever experienced was a robotic duty for the Party.
In addition to being willing to experience human physical satisfaction, she seems capable at first of human thought. She in the Junior Anti-Sex League, yet has consumated with numerous men just for pleasure. She volunteers for Party events, but hates the Party. This passion to subvert the Party encourages Winston and he feels she might be like him.