That is a interesting nuance, but it doesn’t seem relevant considering the answer to your question: “Isn’t there a requirement that one know they are committing a sinful act?” I’m pretty sure the answer is yes, one has to know they are sinning for it to be a sin. The interesting implication is that one could be in an invalid marriage, yet not be living in sin. It does seem a little strange, but makes logical sense to me.
I’m failing to see the logic of “presumed valid” marriage for a person who entered into the marriage in the church without fully being certain of their beliefs, but feeling coerced by their parents and finding it the easiest thing just to go through with it. I suppose one could ask the same question about Confirmation, but I’ve never heard of a Confirmation annulment. I don’t even want to open that can of worms. My question is whether a confirmed Catholic who later loses his/her faith yet goes through a marriage ceremony in the church has a valid marriage? Would their marriage be pretty much the same in the eyes of the church as if they were married by a justice of the peace? I really want to know the answer.