One Man + One Woman = Marriage

Husbands, love your wives, as Christ also loved the church, and delivered himself up for it. (Ephesians 5:25)

But from the beginning of the creation, God made them male and female. For this cause a man shall leave his father and mother; and shall cleave to his wife. And they two shall be in one flesh. Therefore now they are not two, but one flesh. (Mark 10:6-8)