Polyamory

guy breaks up with me saying he's met someone else and he feels he's monogamous, as if this is self-explanatory. wasn't he supposed to not start with someone else if he was so monogamous? why do some monogamists, especially the serial kind, seem to think that breaking up won't be painful for us? how should i feel about having been used as the lubrication to move the guy from one relationship to another? what can i say to him to drive the message home? should i bother?