There are two or more passages in the bible that show us how far our idea of marriage is removed from what God means by marriage:
1. Samaritan woman story
Jesus tells her that she had 5 husbands and the one she was dwelling with at the moment was not her husband.
Q. Does this mean she had 5 weddings (legally wedded husbands) and the one she was dwelling with had not wedded her? I don't think so, logically, it meant she had slept with 5 men and the one she was dwelling with was already sleeping with another woman (whether legally wedded or not)
2. Paul's teaching
Paul says that whoever sleeps with a prostitute becomes one flesh with the prostitute. Two becoming one is God's definition of marriage so in essence, Paul is saying, sleeping with a prostitute is marrying a prostitute.
The biblical idea of marriage is not what we generally have come to believe.