[Albion Wilde]

Parents who are married make the children feel secure. Even if they fight the feeling on the children's side is that they will stay together. Even with a long term shack up that security does not exist.

With co-habiting parents, the underlying example to children is not only that one or both parents does not value the idea of commitment, but that one or both view the other as inherently unworthy. Yet, both are part of the child; hence, the parts that child identifies with in one or both "unworthy" parents are also ipso facto unworthy of love and commitment.

[Harmless Teddy Bear]

Yes, there was some celebrity who went around saying that the reason he didn't marry the mother of his children was that he was not sure he could stay faithful and he "didn't want to make a promise that he might not be able to keep."

There were people praising him for being "honest".

I said he was a jerk.

There is no way you can not keep your promise of fidelity. It is not like catching a cold.

He was surprised a few years later when his shack up honey decided that since he was not willing to commit she was going to leave.

He was also surprised to find out that in law she was considered his common law wife and therefore was entitled to alimony and child support.

IIRC he decided that maybe he should marry her after all.