When a couple performs the marriage act without the marriage bond, any eventual marriage is already on the rocks.
Consider. The most essential element to a healthy relationship is trust. If a couple cannot trust one another to remain faithful, those fears will often become realized. Now any couple that performs the marriage act without the marriage bond has already manifested a cavalier attitude towards the sacredness of marriage. The marriage act outside of marriage poses no problem to these individuals. So when a marriage is eventually contracted between the two parties, how could they possibly trust one another to remain faithful?
Any such marriage has a serious handicap. Thankfully, we are not left to our own devices. God wishes our marriages to be successful, and He is especially attentive to the prayers of the married couple. Please, pray to Him.