According to statistics, forty percent of those who cohabit never marry. But more to the point, of those that do, the divorce rate is fifty to seventy percent higher than it is for those who practice chastity before taking vows. Just as apples and oranges are similar in shape but not in taste, so too with licit and illicit sex. Sin is never a good place to discover the truth. Besides, it is rare that two people who love one another for the right reasons and who are compatible in other ways turn out to have serious physiological problems.
Cohabitation is plain sinful, for lack of a better word.
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.