Apart from the spiritual sanction upheld by all great religious traditions, it is counterproductive from a purely secular standpoint because it undermines self-mastery, the one trait that is absolutely necessary for a good marriage. As such, it requires more virtue and constancy in many respects than consecrated celibacy. Fornication is a training ground for adultery, not marriage, and in the long run, it undermines the self-respect of both parties. We feel good about ourselves . It won't last, and it most assuredly will not lead to what you want.
Sin can only lead, in the long run, to depression, notwithstanding momentary gratification.
Cohabitation saves money . Not in the long run. Divorce, a fairly common outcome, is as costly as anything around. Zacchaeus of gospel fame reimbursed all whom he had defrauded fourfold. And so it will be with fornicators who economize by living in a state of mortal sin.