A google search reminded me that I'm not the first to coin "monogamism".
It's worth noting that (a), dictionaries vary in usages of many or most words, and (b), language is always changing and "evolving". Here's one usage of "monogamist".:
(Sociology) a person who advocates or practises monogamy
Collins English Dictionary – Complete and Unabridged © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003
"Advocate" in the above can also take up various interpretations and kinds / degrees. I could, for example, "advocate" for left handedness while not wishing to impose it on right handed people. Or a right handed person may "advocate" for the notion that all left handed people should become right handed. Someone might even say (i.e., advocate) that left handedness is morally inferior, or that it should be strictly enforced by law or by social custom (again, via shame, guilt, indoctrination, etc.).