Is the Church Not Necessarily Needed by Nature?

I want to throw in here just an objection, or I'm a little skeptical. The church flows from Christ's missionary and salvific character. So if the church isn't absolutely necessary, how much we can say that institution which seems necessary by nature could be symbolic of something which is not necessary by nature? It doesn't seem that in order to maintain that salvation is gratuitous, we have to say that it would be entirely possible for there to be a state of pure nature where rational creatures were not saved from sin. And so then, yeah, there's also the interesting text in the nuptial blessing, right, where marriage is called the one blessing not washed away by the flood