It's a grave matter whether you're aware of it or not, but your degree of culpability depends on a number of things.  Someone who was improperly catechized would be less culpable for entering into marriage invalidly than someone who definitely knew better.

Still, if the Church were to announce that all marriages were presumed invalid and had to be convalidated, the effect is that every married person must now presume he is in a state of serious sin.  Such a person would then be required to live celibately until a marriage could be contracted validly.  The amount of damage that would do to the spiritual lives and consciences of the faithful would be massive.