Does the act of sex constitute marriage in God's eyes? I came across this verse today (bold added):


  1 Corinthians 6:15-17 (NASB)
  15 Do you not know that your bodies are members of Christ? Shall I then take away the members of Christ and make them members of a prostitute? May it never be! 16 Or do you not know that the one who joins himself to a prostitute is one body with her? For He says, “THE TWO SHALL BECOME ONE FLESH.” 17 But the one who joins himself to the Lord is one spirit with Him.


In light of this verse:


  Mark 10:7-9 (NKJV)
  7&nbsp; ‘For this reason a man shall leave his father and mother and be joined to his wife, 8&nbsp; and the two shall become one flesh’; so then they are no longer two, but one flesh. 9&nbsp; Therefore what God has joined together, let not man separate.”


Does this imply that the act of consensual sex initiates a marriage in the eyes of God?

Note: My doctrinal tradition is protestant.
No, for a whole lot of reasons.


Old Testament Law says that a man who has sex with an unmarried woman has to marry her. That would be unnecessary if having sex made them married.
If that were the case, some teacher somewhere would be reminding people that they had to treat the people they had sex with as if they were wives.
Tamar has sex with Judah because he won't consent to give her to his son in marriage. After they have sex, there is no thought that they are married. Judah relents and allows Tamar to marry his son.
As pointed out elsewhere, fornication and adultery would both be non-existent events if the act of sex made you married.
David has sex with Bathsheba, and then later marries her. Also unnecessary if the sex made them married.


Having sex "makes you one flesh", and that is something that should be reserved for marriage. 

As for Mark 10: "a man shall...be joined to his wife, and the two shall become one flesh". That's two things that should happen, not one.