In 1984, it is impossible for Winston and Julia to be seen together in public because their relationship is not sanctioned by the party. In fact, the party actively encourages people to become celibate and is, as Winston comments, "actively trying to kill the sex instinct." (Part 1, Chapter 6). The party does, however, condone marriage, though strictly for the purposes of procreation, though does not allow people to choose their own marriage partner. This is a decision for a specially-selected committee. Anybody who contravenes these rules and regulations attracts the attention of the Thought Police and the very real possibility of going to Room 101 or, even worse, being vaporised.
For Winston and Julia, then, being discrete is of paramount importance. Their first meeting takes place in the busy Victory Square (Part 2, Chapter 1) but the fear of being detected is overwhelming. From then on, they meet in places which are far away from telescreens and prying eyes. As such, future meeting places include the woods, an abandoned church, and, finally, in a rented room above Mr Charrington's shop.
While they cannot avoid the Thought Police forever, this cautionary behaviour enabled their relationship to thrive and drives the plot to its grand, yet tragic, conclusion.