... Because when the pressures that society puts upon either sex to act in certain ways (i.e. women should seek marriage to validate their existence, men should seek female romantic partnership in general to prove their masculinity, and with the expectation being on men to prove their commitment via proposing marriage due to the assumption that their motivation is lesser) are equalized as stereotypes are hopefully discarded, then the two parties will likely be more concerned with their actual relationship than what "step" in the relationship "ladder" they are on.

As a result, marriage will hopefully become more of a formality; the logic being that it is only justified to enter into such a binding contract when both parties have experienced enough of life and its ups and downs to gauge the others responses to any crisis, grown as people through a number of individual character-altering periods without growing apart, and in general have developed such a profound and inseparable relationship that marriage is almost redundant.

Whether or not the act of getting married has actual weight, I imagine marriage will eventually lose its magical nature and people will just get bored with the idea, except for its legal benefits.