eros does not last long into a relationship: sooner or later, either the relationship dies or agape begins. I think Agape starts when people have to start forgiving each other for wrongs done or merely for perpetually annoying habits.
The only difference between monastic life and married life is that whereas married life starts with eros, monastic life has to start straight off with agape: initially more difficult of course but probably rather easier in the long run.