Thankfully, this stage in a relationship is only temporary. When the haze dissipates, only the strong relationships survive. When a couple lives together, the comfort level between the two gradually increases until nothing is sacred. Little quirks that a person tries so desperately to keep hidden eventually come out into the open. Living together before marriage clears the air and reduces the possibility for major surprises that could lead to marital strife.
Sex is another issue that tends to cause problems in marriages.
Of course people can have sex and not live together, but the sharing of a bed each and every night certainly facilitates the activity. Sexual compatibility is extremely important in the satisfaction and happiness of a marriage. I lived with a man for four years before the relationship ended. Had we married, I would be divorced right now. I can provide no more substantial proof of living together reducing the occurrence of divorce than that fact alone.