We’re on the same page. I think one is marriage and the other is legal union. And the latter can also be defined as legally defined serial monogamy.
And when one enters into the latter with that understanding, prnupts may become more of a significant thing. Frankly, it is very much a legal version of “making an honest woman out of her”. There is nothing spiritual there, by definition.
FWIW, for tax reasons my wife and I seriously considered getting married, but never involving the state in it. That is, legally we would still be single people cohabitating. We both take the spiritual commitment VERY seriously, however.
I wonder if, for example, the Catholic Church would marry two people under Canon Law without a state-issued marriage license. I mean, it seems to me that the secular institution stands in so many ways athwart the Catholic understanding of marriage that one would think that the Church would prefer to avoid any association with that institution in the public mind. I think that other Christian denominations might feel this way. If memory serves, the great R.J. Rushdoony (my favorite Protestant theologian) had something to say about this.