This is a bit of a pet peeve of mine, mostly that it is so common in the vernacular.
Polyamory: many loves
Monogamy: one partner*
Why are these always used as opposites? They may be almost mutually exclusive (not going to go into when they aren’t because that’s just semantics) but they are not logically linguistic opposites. If one looks at roots (both Greek and Roman in the case of polyamory) the opposites are as follows:
Opposite of many loves, is one love–monoamory
Opposite of one partner is many partners*–polygamy
I propose the increased usage of the word monoamory–despite its plethora of vowels, it is the technically correct word to use opposite polyamory.
*: I simplified the terms incredibly. The ending -gamy usually refers to marriages, but is often used to also refer to non-married relationships as well.
Purple Penguin 
Polyamory, I believe, is a relatively new phrase (which would explain it’s lack of a linguistic antonym). It is weird, though, that for so long monogamy has been used to refer to relationships in general, while it’s opposite, polygamy, was used only for marriage specifically.
Yes, “polyamory” is a very new phrase–most sources have it coming into usage in our lifetime (in the early 90s to be accurate). Can’t find a specific source this late at night–if needed I could later.