I actually was thinking about this the other day. I was wondering how the word monogamy, with it's -gamy ending, is applied to all dyadic relationships whether married or not, yet polygamy only refers to plural marriages. Maybe it's because Western culture just expected anyone who pairs up to eventually marry. Dating and not marrying is a fairly modern development. I meant to look up the etymology of -gamy and how it relates to marriage, but then forgot.
So, here's what I just found now at etymonline.com:
comb. form meaning "marrying," from Gk. gamos "marriage" (see gamete) + -ous.
"sexual protoplasmic body," 1886, name introduced in Mod.L. by Austrian biologist Gregor Mendel (1822-1884), from Gk. gamete "a wife," gametes "a husband," from gamein "to take to wife, to marry," from PIE base *gem(e)- "to marry" (cf. Gk. gambros "son-in-law, father-in-law, brother-in-law;" Skt. jamih "brother, sister," jama daughter-in-law;" Avestan zama-tar "son-in-law;" L. gener "son-in-law"). This also is the source of the suffix in monogamy, etc. The seventh month of the ancient Attic calendar (corresponding to late January and early February) was Gamelion, "Month of Marriages."