What I always heard was that essentially monogamy was divided into two subgroups:
- having only one partner ever, for life;
- having more than one partner in one lifetime, but only one partner at a time.
Anything falling under the second category could technically be called serial monogamy. It seems safe to say that most monogamists are "serial monogamists," but not all.
The main negative usage I've heard is when some monogamous person trash talks polyamory, and some polyamorist reacts by pointing out that most monogamists are "serial monogamists" (that is, they do have more than one partner over the course of one lifetime).
I prefer not to get involved in that kind of argument. If some monogamous person wants to look down on polyamory, that's their choice, and arguing with them isn't likely to change their mind. Worse, the "serial monogamy argument" is just switching the shoe to the other foot, so that now polyamorists are looking down on monogamy. That's not a good solution to the argument, IMO.
A classic example of serial monogamy (especially the negative kind) would be someone who married and divorced many times. But from my understanding, anything that involves more than one partner over one lifetime (but still always one partner at a time) could technically be classified as serial monogamy.
Since the word is often used in a negative or argumentative context, I usually just don't use the word. But the above paragraphs tell something about what definitions I've heard for it.