To be fair, Jesus defined marriage as being between 1 man and 1 woman.
But you're definitely correct that there was rampant polygamy in the Old Testament.
To also be fair, the 'definition' you're referring to is in Genesis 2:24, which is as you know - is the first book of the Old Testament.
As the story goes... And someone correct me if I'm wrong but I don't think Jesus ever said
that. In fact, the inspiration of the words in Genesis were widely (but not completely) attributed to Moses who never even met Jesus. I think Moses predated Jesus by ~2,000 years. Maybe my time lines are wrong here or maybe it's God that inspired Moses' or maybe Jesus and God are being used interchangeably.
Nonetheless:
"The first thing we need to be honest about is the Bible has no one clear definition of marriage. For example, the practice of polygamy was common in the Hebrew Scriptures. King David, a “man after God’s own heart” had as many as eight wives (see 1 Chronicles 3) and other prominent Hebrew figures also had multiple wives. (I’m not including Solomon’s seven hundred wives here because an argument can be made that he did indeed break a Deuteronomic command for a king not to have
too many wives — see Deut. 17:17.) Like it or not, polygamy is one “Biblical definition” of marriage.
Some argue that Jesus himself, echoing Genesis 2:24, defines marriage as between one man and one woman in Matthew 19, “For this reason, a man shall leave his father and mother and be joined to his wife, and the two shall become one flesh” (Matt 19:5). While I don’t think Jesus’ intent in this exchange was to define marriage, even if I did, the practice of one man, one woman is simply one more definition of marriage in the Bible, not the only definition."
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