I thought marriage was a sacred union not a contract.
Spirituallly or whatever, yes it is. However, legally, you are still entering a contract. Orally you are saying yes, I will blah blah blah you for the rest of my life...in addition to that, you sign documents stating that this promise took place a certain date and witnesses present sign also.
Even though a lot of people create their vows these days, there are certain phrases that must be said in order to make it official.