Dittoes. My husband and I lived together for around three years before marrying, and next month will be our 21st anniversary. It's better now than it's ever been despite the absolute worst things that have happened to us and our children. When we shacked up we weren't even thinking "I'm out of here if it goes bad" - it was more like "THIS is the one I want to be with."
That said, I do believe that many shack-up situations do have that fair weather mentality built into it.
That's really the point.
What is a marriage? A marriage is a lifetime commitment between a man and woman for both the begetting and raising of children, and the mutual care of the spouses. Even in sacramental marriages, it is the couple that confers the sacrament, rather than the priest or minister, who acts only as a witness for the church/Church.
If a couple cohabitates with the intention of remaining together for life, the couple would have entered into a natural marriage in everything but name.
If a couple cohabitates or even participates in a wedding ceremony with the intention of separating in the event of difficulties, then the couple has not entered into a marriage.