Where in the Bible is it first mentioned marriage should be a union of one man and one woman?
Luke 14:7 - 14
ESV - 7 Now he told a parable to those who were invited, when he noticed how they chose the places of honor, saying to them. 8 When you are invited by someone to a wedding feast, do not sit down in a place of honor, lest someone more distinguished than you be invited by him.
For follow-up discussion and general commentary on the topic. Comments are sorted chronologically.
Todd Rushing
Genesis 2:24 is the first mention of marriage. "For this reason, a man shall leave his father and mother and shall cleave to his wife, and the two shall become one flesh." Since the Bible goes on to teach about sexual sins, it seem that this reference in Genesis (and again in Mark) would indicate marriage as one man and one woman. Wife in this context is a feminine descriptor.