The amount if paperwork required to get married is quite small, compared to something like buying a car. The overall cost of getting married is small. The signal if one person in the relationship refuses to get married is probably strong. If both people refuse to get married it is a non issue, but I suspect that is still not the norm.
The cost of getting married isn't small when you include the probability and cost of divorce. (Or the consequences of not getting a divorce when everyone is miserable and fighting)
Those costs are associated with a civil union. In most western countries, and in many states, having had kids together and/or a joint bank account is enough to have the same divorce consequences in separation.
Only 9 states still have common-law marriage, most that have historically recognized it have since thankfully abolished it.
Of course if you have kids, you still have to worry about child support, which can be very problematic in several states, but that is a whole other can of worms.
In the country I now reside in, there is little difference between common law marriage, "official" marriage, and civil union - if you have no kids, and divorce/separate within 7-8 years, then each partner gets what they brought in (and any substantially "joint" income during the same time, e.g. appreciation of an apartment, unless otherwise agreed upon in advance, is split 50/50) . If you are married and/or crossed to the 15-20 year area unmarried, then any income generated during the "united" period (including retirement accounts, pension funds, etc) is assumed 50/50, but anything before (and inheritances) go back to whoever brought them in.
Prenups, of course, take precedence although some have been nullified by courts as being unreasonable (mostly in cases where the prenups says in the event of a divorce wife gets little/nothing, but then separation comes e.g. after 20 years of marriage/union and giving up a career to raise 4 kids with husband's approval and support).
