Umm… no. I don’t think that’s correct.
Not so during a Natural Marriage.
The difference between a valid natural marriage and a valid sacramental marriage is that the latter is a source of grace for the couple, not that they’re changed ontologically.
Think of it this way: what happens during the sacraments of initiation is that the universe is changed FOREVER. What existed 5 seconds prior, exists no more.
Yes – baptism is permanent and unchanging.
Similarly at ordination. What exists 5 seconds prior, exists no more. The man who is ordained is changed FOREVER, ontologically, at his very soul. He is now a priest. 5 seconds ago, he wasn’t.
Yes – ontological change.
And that’s what happens at marriage.
No, because marriage lasts only “until death do us part.” Widows and widowers aren’t changed back, ontologically speaking, to their pre-marriage self. And, of course, widows and widowers are able to enter into marriage again.
It’s not an ontological change that one experiences at marriage, so much that it is a vocation that is grace-filled, and which images the relationship of Christ to His bride, the Church…