It all started before Christianity was established. In fact, it started with the Greco-Romans. They placed anti-polygamous laws to prevent elite men from acquiring multiple wives, which allowed lower ranking men to get wives of their own.
The ancient Greco-Roman and medieval European leaders who embraced anti-polygyny laws were heavily invested in the business of war, and their own social status and indeed survival often depended on their ability to maintain large, well-funded armies. And the imposition of monogamy produced bigger, better armies, because monogamous groups can grow larger than polygymous ones.
Modern culture tells us that each person has their "one," a "perfect partner" that would share their lives with.
Although polygamy is expert in various cultures, humans yet tend toward monogamy. But this was not always the norm along along amid our ancestors. Other primates -- the mammalian organization, to which humans belong -- are still polygamous, too.