The word is built from two pieces: mono- (one) and -gamy (marrying). One way to hold it is a single line drawn between two beings, and no second line leaving either of them. Whether that line is a marriage certificate or two birds returning to the same nest is what the two senses differ about; the picture of one line is the same in both.
這個字由兩塊組成：mono-（一個）和 -gamy（婚配）。可以想成兩個個體之間畫了一條線，而且沒有第二條線從任何一邊拉出去。這條線是一張結婚證書，還是兩隻鳥每晚回到同一個巢，就是兩個意思分開的地方。但「只有一條線」這個畫面兩邊都一樣。
From the word’s history
Wiktionary gives monogamy from French monogamie, from Late Latin monogamia, from Ancient Greek monogamía, and analyses it as mono- + -gamy. The modern word still says the same thing: one marrying.
字源資料說 monogamy 經法文 monogamie、後期拉丁文 monogamia，來自古希臘文 monogamía，並把它拆成 mono- 加 -gamy。今天的意思和當初一樣：只有一個配偶。
Wiktionary, "monogamy"
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