So the term 'serial monogamy' developed as a way of trying to address the ambiguity of the length of 'relationships'.

People like me (ie old farts) wouldn't call 3 sexual partners in a year as monogamous. The element missing for me is the notion of an extended commitment - extended meaning greater than a year.

I guess these terms developed in a time when people got married and remained faithful for the entirety of their life, much like wild animals that mate for life.