This thread is getting a bit old but had to add my tuppence, I really struggled with this before I got married, when I counted myself a Christian. When I moved in with my then fiance, a number of our friends were told not to speak to us any more by our church's pastor, and my fiance had a visit from his mum's pastor telling him he would be living in sin if he went ahead and moved in with me. This was all in the shadow of a pretty desperate situation where we had no other option for living arrangements (none of them stopped to check that first funnily enough and we weren't actually sleeping together at the time anyway!)
I think it all depends what you define as marriage, and I think marriage itself has a dodgy past. It wasn't until the Council of Trent in the 1500s that a marriage was defined as beginning with a priest and two witnesses. Before then it was pretty much move in together and common law marriage applied.
Of course you have to pay a priest for the privilege so you're essentially buying your way to guilt free sex.
I think marriage is more fluid than that, and definitely can't be defined with a specific beginning point at a marriage ceremony. What if you don't have the money to get married? (even the basic certificate costs money, not to mention any celebration costs). Also there is so much work to be done before a ceremony in becoming two souls intertwined.
So I don't think a marriage service/certificate defines when you're married. I think it's between you, your partner and God to decide, and obviously if you're a Christian your decision would be taken in light of the rest of the commandments/Jesus' teaching etc - i.e. don't take the mick!