Yet no one complains when we use the term monogamous to mean sexually exclusive: committed sexually to one person, even when you're not married. Gamos means marriage in Greek. So monogamous means married to one other person. Yet, when we are dating, if we are seeking a partner with sexual exclusivity in mind, we say we are looking for monogamy. We can be looking for "monogamy" and not actually be looking for a legal spouse.
I once started a vocab thread saying these kinds of people should call themselves mono-amorous, but no one seemed interested in discussing it. (Maybe because queer rights weren't involved, and it just seemed like pedantry [excessive concern with minor details].) Monogamy is not the opposite of polyamory. The opposite of polyamory is mono-amory. The opposite of monogamy is polygamy (married to more than one, which is illegal in most countries, but people who practice it without being legally married to more than one, declare they are in polygamous marriages in the sight of their god, or whatever).
I bring this up, as I did the term "polyam" in my previous post, to highlight my opinion that I'd prefer to share neologisms with other groups, who many define it differently, despite complaints. Words have different meanings all the time. And there are words for this issue: homonym or polyseme.