I have to agree that yes, in Biblical times, sex was what often made the marriage "official". You had sex, then you were married. But the way they did things in their culture was different, so we can't make the exact comparison here. Do I believe it is this consummation that makes us married in God's eyes? Yes, I do. However, because legally I am not married, and I also believe we are told to submit to authorities, I would not have sex outside of marriage because I am not legally married. This law is not at odds with God's law, and actually provides us more protection if we stick with it. Some people will say, "Well, they PLAN on getting married. They're engaged!" That's wonderful that they are planning a life together, but there are many examples of relationships falling apart even at that stage. Some people might argue, "Well, divorce is a possibility, too!" Well, yes, but that's another issue entirely. If you're not married, God doesn't see a problem with severing a relationship. But Jesus mentioned divorce specifically, and said we are only to divorce for marital unfaithfulness. People divorce for many other reasons, and thanks to God, who forgives us and now we live with no condemnation. But, back to divorce, if you haven't ever married and yet you have sex, and then end the relationship, in God's eyes it really is no different. And, as has been pointed out already, a relationship that intimate creates strong emotional ties. That is why God intended us to share it with one person only, and why it causes so much trouble if we choose to share it with more than one.
Besides, every time I've seen an example of someone having more than one wife in Scripture, it's nothing but a headache for the guy. Honestly, could you imagine being married to both Rachel and Leah, competing and fighting with each other to have more children than the other, to win your affections more? It'd probably be like referreeing children themselves. And Sarai and Hagar? Insane jealousy and hate - Sarah made Abe's life miserable after she TOLD him to sleep with Hagar. Really, it's just not worth it. Lol.

(Trying to lighten things up a bit here at the end; I hope you will excuse my lame attempts at humor.)