Let me ask this, because I think I get what you're saying with the 'one flesh' deal, but I want to make absolutely sure what you mean. Because the way I understand it is that when you have sex, you've become one flesh with that person, and like I had said earlier is that I've done this with 3 girls total in my life.
What is your view on what it means to be 'one flesh' and can it happen outside of marriage? Not meaning can it as an 'allowable' term, but an 'is it possible' term.
Let's start at the beginning. God made man and woman to be one flesh, and that's the basis for marriage. In this one flesh relationship, man and woman are together made in the image of God, male and female as separate expressions of the one image of God. When Adam sees Eve, he says, "You are flesh of my flesh, bone of my bones". I think "one flesh" goes beyond sexual intimacy, but sexual intimacy is certainly a big part of it.
That's the ideal God made for marriage. We haven't always lived that way. Paul explains that having sex with a temple prostitute is not OK because you are becoming "one flesh" with her. That doesn't mean you are then married to the prostitute, or that you have the full intimate relationship with the prostitute that God intended you to have with your wife. In the 1 Cor 6-7 passages, Paul's main concern is actually that we keep ourselves holy to God, as temples of the Holy Spirit, and we can't do that by becoming one flesh with temple prostitutes.
Some people have argued that this means you are married to anyone you have had sex with. The Bible doesn't seem to treat it that way. For instance, in Genesis 34, Shechem has sex with Dinah, but they are not treated as married after that point, and Shechem asks for Dinah's hand in marriage. So I don't think that just having sex with someone means you are married to them. I've heard this taught before, and I actually used to believe this for a while when I was much younger, but I no longer believe it.
On the other hand, in at least Genesis 24:67, the basic way to get married was to have sex:
Genesis 24:67 Isaac brought her into his mother Sarah's tent, and took Rebekah, and she became his wife. He loved her. Isaac was comforted after his mother's death.
The Bible doesn't tell us about marriage licenses and weddings and wedding dresses and rings. That's OK, we have our own culture and time, and we can do it our way, but when we deal with other cultures we have to be careful not to think of our own culture as normative. I think any time that a man and woman see themselves as married, then have sex, you've got a marriage. But I also think there is great wisdom in having documentation, witnesses, and the support of your community, all things that happen in our modern wedding ceremonies.
This raises interesting questions if you have a sexless marriage. In Catholic canon law, I think they distinguish a marriage that has been consummated from one that does not. And to Catholics, it's not the wedding ceremony that consummates a marriage. (I'm not Catholic, not terribly conversant with canon law, and may have munged this, but I think I got it right.)
Hope this helps,
Jonathan