Biblical scholars Dr. Robert R. Cargill, Dr. Hector Avalos, and Dr. Kenneth Atkinson, say this is due to “a lack of biblical literacy.”
“We wish to clarify that the biblical texts do not support the frequent claim that marriage between one man and one woman is the only type of marriage deemed acceptable by the Bible’s authors.
The fact that marriage is not defined as only that between one man and one woman is reflected in the entry on “marriage” in the authoritative Eerdmans Dictionary of the Bible (2000): “Marriage is one expression of kinship family patterns in which typically a man and *at least* one woman cohabitate publicly and permanently as a basic social unit” (p. 861).
In fact, there were a variety of unions and family configurations that were permissible in the cultures that produced the Bible, and these ranged from monogamy (Titus 1:6) to those where rape victims were forced to marry their rapist (Deuteronomy 22:28-29) and to those Levirate marriage commands obligating a man to marry his brother’s widow regardless of the living brother’s marital status (Deuteronomy 25:5-10; Genesis 38; Ruth 2-4).
Others insisted that celibacy was the preferred option (1 Corinthians 7:8; 28). In fact, during a discussion of marriage in Matthew 19:12, Jesus even encourages those who can to castrate themselves “for the kingdom” and live a life of celibacy. Ezra 10:2-11 forbids interracial marriage and orders those people of God who already had foreign wives to divorce them immediately.” (1)
Perhaps this meme, though oversimplified, is relevant