Well, first off, marriage is not strictly related to religion. People have been getting married before Christianity even existed for example. Non-religious people usually don't have a religious ceremony in a church, they have what's called a civil ceremony. And they can chose where they have this ceremony, ei. City Hall, outdoors etc.. So they don't employ the church's services at all. I don't know where you got the idea that marriage has to go through the church.
Second of all, from your post, I'm going to assume that you don't know that marriage is actually a legally binding contract that the government recognizes. There are a couple benefits to this: like certain social services, certain tax breaks, also can make a joint bank account that can come with lots of other financial benefits.
And thirdly, and this is probably the most important point of marriage, is that it's a commitment. When you marry someone, your basically telling them in a fancy, and elaborate way that you will love, and support them, and be faithful to them for the rest of your life. If you think of marriage in a cold, rational way, then you will never understand. You have to use your emotional side. I really hate to pull the age card, but how old are you? It's likely that you'll understand this concept when your older.