You do not need a Nobel prize for biology to realise that people of opposite sexes are attracted to one another and that this has something to do with the propagation of the species. In fact, it goes much further than that. Compared with other species, human babies are born prematurely and are totally dependent on their parents for years - let's not say how many. This dependency is demanding and requires both parents. In nature, the child of a single parent would be unlikely to survive unless surrounded by a very supportive community. So biologically, the attraction between the sexes is much deeper than a passing fancy and marriage celebrates that fact.
But there is much more to marriage than the mutual support necessary to bring up children. The mutual trust which grows out of falling in love can create something which is more than just two people living together. A couple can do more, they can achieve more, and in the process of adapting to each other, they both become better individuals. That is the possibility that marriage offers and this is why we regard it as a God-given goal to aim at.
But it is dependent on total commitment. If you are committed to one another, no ifs or buts, you are married. If not, you are not. The commitment, and flowing from that, the desire to make the relationship work, is what marriage is all about. Marriage is wonderful: don't be content with anything less.
After birth and death, marriage is the most important event in your life. A wedding in church can both help the marriage and be a wonderful celebration of a joyful event. It does help commitment to make solemn vows and to do it before God. Wedding vows are made before God in a Registrar's Office quite as much as in church, but making them before the altar impresses that fact upon us much more forcefully. "For better or worse, for richer or poorer, in sickness and in health, 'til death do us part". It's hard to say that in church and not realise what you are taking on.
In church too you are following a long tradition. For centuries, young couples have to come to our church to be married, cheered on by friends, family and the rest of the village. Being married in church is symbolic of taking your place in society: it's something to be proud of.