I think the word "serial" is this problem. It conjures up a picture of regular episodes in relatively quick succession which isn't really what you guys are talking about.
A fellow blogger did quite a nice post about the seasons of a relationship: from spring through to winter. My take on this is: spring full of promise and beauty; summer ripe and luscious; autumn being the harvest but also getting a bit rougher and winter giving yourself the choice between cold and bleak or wrapping up cosily by a fire together in harmony. In reality the winter of a relationship is a bit of both.