1 Corinthians 6:15-18  . . . Shall I then take the members of Christ, and make them members of a prostitute? May it never be! Or don’t you know that he who is joined to a prostitute is one body? For, “The two”, he says, “will become one flesh.” . . . Flee sexual immorality! “Every sin that a man does is outside the body,” but he who commits sexual immorality sins against his own body.

Matthew 19:5-6  . . . ‘For this cause a man shall leave his father and mother, and shall join to his wife; and the two shall become one flesh?’ So that they are no more two, but one flesh. What therefore God has joined together, don’t let man tear apart.

[I believe this last statement means “what God declares or regards as being joined together . . .” – not necessarily a union that took place in holy submission to his will. Verse 5 suggests that God's plan was for sex to occur only after the deliberate and public act of leaving one's parental authority for the purpose of joining oneself to other.]