In my opinion, the two of them are not really in love with one another. Winston may be more in love than Julia, but they are really both just using the other to fulfill a particular need.
Julia is just using Winston because she hates the Party having control over her personal life. She just wants to have some fun and she does not really care all that much about whom she has fun with. All she really cares about is her own desires and about hurting the Party.
Winston is looking for a bit more than that. He seems to truly want to have a relationship. But it is not clear to me whether he really wants a relationship for its own sake or because such things are forbidden. He feels that what the Party is doing is wrong and he wants to try to destroy the Party's power. I do not know how much he actually cares for Julia in particular and how much he is just having the affair because it is a human relationship that he longs to have.