lambslove, I have just read a book called eros redeemed by John white, I suggest you read it it might help.
But if you want some information right now;
one of the key elements of marriage is sex. that is how a man and a woman become one flesh again as they were before they were separated when God created woman out of adam's rib. Therefore this state is where man and woman best reflect God, it is a very holy thing and some would say that it is the most innocent and beautiful form of worship when the holy Spirit joins you together in a way that glorifies God. It therefore is a vulnerable thing and must be protected, so before you have sex you must be joined before God by his spirit. This is the importance of marriage. (I will go on to the procreational side in a mo). This is why it is so attacked by Satan, the last thing he wants is for you to Glorify God in this way.
If you have sex outside marriage it is like facing a hostile army naked on the top of a hill, you can be exposed to all kinds of demons and schemes of the enemy aimed at driving a wedge between you and your partner. Entering into a marriage contract before God is like entering into a strong stone fortress, you are protected by Him.
As far as children go, that is a key part of both marriage and sex the main point of getting married in the past was to create a safe place for children to be raised. If you live together and raise a child without being married in the sight of God, it is like putting a child on the top of a mound of rubble rather than inside the stronghold and expecting it to be safe from the marauding army! No parent wishes this obviously!
so what makes the difference? well God forms covenants with his children, he formed a covenant with Abraham which on his part was him committing to all the promises hed given. In marriage He is commiting himself to you as much as you are to your partner, the best marriages are always where both memebers focus on God first and their partner second, God is the unifying factor he sits in the middle between you, in a secular marriage it is only twofold - you and your partner and not threefold, which is part of the way it reflects God, by mirroring the trinity.
In the old and new testaments the covenant was made with blood, and that is why the hymen is broken on the first night, if you have slept with someone else many suggest that in the sight of God that is your husband... but he is a God of grace and for those who have that problem healing is free and available! So why have a ceremony if the first night counts? Well.... Id want to be sure, I wouldnt want to risk exposing my marriage to Satan right from the start, thats a recipe for disaster... but likewise there is healing and freedom available from God for anyone who has this problem.
I think Ive given a slightly different view to those above so maybe it goes closer to answering your question. another good book is "the ancient paths" but right now I cant think who its by, that is where the hilltop analogy comes from, if you want to know I can find out for you.
God bless you and your fiancee
amatire