One could argue that the point of no return for David came when he decided to sleep with Joey. Yet, after that, David still convinced himself that he could escape the fact that he was gay. He went all the way to Paris under the pretext of finding himself when all he really wanted to do was to escape himself. He gets away with the charade for a while. He even proposes to a girl, Hella. But when David meets Giovanni in Guillaume's bar everything changes. He keeps telling himself that there has been some sort of misunderstanding, but when he finally sleeps with Giovanni he feels nothing but relief. By then, the cat's out of the bag and it's only a question of how things will resolve themselves.
Here we've passed the point of no return: David has already decided to sleep with Giovanni. Yet somehow David and Giovanni continue to function in a sort of dream state where they imagine that things will just work themselves out. David often refers to Giovanni's room as a sanctuary, a place where time does not seem to operate. When Hella arrives, David simply abandons Giovanni and begins to try to make a life with Hella. The entire time the reader knows that conflict is forthcoming, but it's not clear exactly how things will resolve themselves.
It's hard to pinpoint exactly where the tragedy of Giovanni's Room begins. For David, perhaps it began as far back as when he abandoned his boyhood love Joey. Perhaps it began with the death of his mother. For Giovanni, the tragedy began when his wife gave birth to a still-born child. Within the timeframe of the story, though, there still seems to be a prospect of happiness up until David and Giovanni's falling out. As David sees it, he has to leave Giovanni. Yet, after he does, he keeps tabs on him out of worry. He can tell that Giovanni is falling down the social ladder, but things don't really take a decisive turn until Guillaume winds up dead and Giovanni goes to trial. From then on, David's life is a downward spiral of despair, and one tragedy follows closely on the heels of another until the very end of the book.