A descriptor for a person who has commitment issues but does not engage in cheating or infidelity. A serial monogamist likes the emotional and physical intimacy of relationships and therefore seeks partners who want a longterm romantic relationship. A serial monogamist may or may not warn their partner about their fear of commitment. (Often the partners foolishly believe they will be able to change this about the serial monogamist.)
The relationship may be short-lived or it may last a few months to years, however the serial monogamist is always holding back and if the other partner in the romance pushes at all, the serial monogamist will end the relationship swiftly and often without emotion.
Serial monogamist: I broke up with my boyfriend.
Friend: What???? You guys have been together for three years!