For me, marriage is more about a life long commitment than it is about love. Love is important but in many relationships it is fickle.
A life long commitment, that's sincere, true and constantly renewed has staying power through the ups and downs, and can remain even when the love seems like it isn't there.
Love is an emotion and emotions can change based on circumstances but the goal with marriage is to state that you will remain together regardless of the changes that occur.
If a person is against marriage I can imagine that they might want to do something else to state their life long commitment, but I don't see a need to invest a new method of stating a life-long commitment when we already have marriage.
*I don't see a need to INVENT a new method