well, yeah, just as a person can obtain intial sanctifying grace in extra-sacramental way through baptism by desire, I believe the Church recognizes the possibility of marriage by desire, in the circumstance in which, if two persons are able to enter a valid marriage, and the cirrumstances are such that no reasonable access to a priest is possible, they may be joined in marriage. Remember, it is the man and the woman themselves that confer the sacrament on each other. The priest is but a witness. He does not confer it. But, of course, in like 999,999 cases out of a million, the formal ceremony of the Church, with a priest present, is required.
Keep in mind, not that it would ever happen, but even if all priests and bishops were murdered in the world, we would still have the sacraments of baptism and marriage, and it is for this reason that most heretical communities (Protestants) have , and can only have up to, 2 sacraments, baptism and marriage.