If you believe that marriage has an important legal function, it's also not fair to preclude polygamists from getting those same legal utilities.
A much better generalized solution is to expand the purview of contract law to allow it to do everything marriage does.
The government does recognize a legal "coming of age" in several cases (at least in the U.S.).
- Driving a car (restricted at 15 years and some months, open at 16)
- Entering the Majority (18)
- Can enter the military (18, or 17 with some restrictions)
- Can have Sex (rules vary by state, but is generally 16-18 with various limits)
- Can drink (21)
etc.
It just so happens that these legal coming-of-age points don't specifically align with any religious coming-of-age. The two are divested from each other due to various pre-existing cultural forces. But it's completely conceivable in some alternate universe that the two are conflated and one enters the majority legally at the same time as going through a "traditional" Sacrament of Confirmation.
Before most popular religions were created, marriage was already old.
For cultural reasons, it's popular to do a RELIGIOUS CEREMONY along the marriage. What's religious there is the ceremony, not the marriage.
You can have religious ceremonies without marriage, and marriages without the religious ceremony. Two independent concepts that, by happenstance, often come together.