Throughout Giovanni's Room, David tends to avoid making decisions. He is smart enough to realize, however, that not making a decision is just another way of making one. The problem is that he seems to always think that he has two false options, and that it doesn't matter which one he chooses. David would like to be able to choose who he is attracted to (he wishes he were attracted to women) and why, but he cannot and the result is that he regards his choices as insubstantial and unimportant.
David always uses the same metric to make decisions: he chooses the path of least resistance. In this sense, he never actually decides to leave Giovanni, he simply realizes that, for the time being, it would be easier to promise to marry Hella.