I'm not married to my son's mom yet. We just haven't gotten around to it because:
we're not religious
marriage as an institution just doesn't mean much to us outside tax/other legal benefits
proper weddings, then kind you can invite extended family to, are prohibitively expensive
our son is developing just fine without us being married
We'll get around to it eventually, maybe. It's just not a priority right now.
That said, I hear things can get unpleasant for unmarried couples in the United States when something unpleasant happens and the partner is not allowed to inherit, visit their loved one in the hospital, and so forth. They're treated like any other acquaintance.
Many European countries will grant the same rights as a married couple to a couple that has been together (but not married) for a long period of time and shares property and financial assets. Might was well get a quick civil ceremony in those circumstances though: that would probably make life easier.
Germany treats marriage entirely as a civil matter. The official marriage is performed in a municipal office by a municipal official. If the couple wants to, they can then have a religious ceremony, but the state doesn't regard a religious ceremony (at least not one performed within Germany) as a legal marriage. I think that nicely highlights the fact that the marriage as regarded by the state and marriage as regarded by religion are really two separate things that often happen to be co-mingled.
And if you make a will, you can pass on your property to anyone you want. Visiting someone in the hospital may be an issue, but there are legal documents like health care proxies that enable you to designate someone to make medical decisions for you if you can't do so yourself.