I'm still not very clear on this.
"Since for us a marriage is something that God does through the Church, without contractual elements, that would be the same as saying that someone who walked up for communion who hadn't been baptized and "slipped through" didn't receive the Body and Blood of Christ, since his reception of communion was grossly defective."
Right, I understand that the Body and Blood of Christ are still the Body and Blood, even if the person receiving is unbaptized, ignorant, unbelieving, in serious sin, etc. But that's because the Holy Spirit, through the action of the priest (the minister of this sacrament), has caused the bread and wine to actually become the Body and Blood of Christ. Whether the recipient knows it or not, the reality (Christ) is there.
But in Matrimony, the bride and the bridegroom are the ministers of the sacrament. Are they not? Without their true consent, Matrimony doesn't "happen." To create an exaggerated example, you can't slip a girl a GHB, walk her through an Orthodox wedding ceremony, and then claim that she is actually married merely because the ceremony was technically complete.
Both the bridegroom and the bride have to intend what the Church intends.
Thus, similarly, in the examples I gave before, one party is not intending to marry (as the Church understands marriage) because he or she is practicing fraud or deception and is actually unwilling to enter into a lifelong, exclusive union. Thus there was a lack of consent; and consent is essential to a sacramental marriage, isn't it?
Understand, please, that I am not arguing with you here. I just want to learn how the Orthodox Church sees these things. If I seem a little slow, I am not being obstinate --- I'm just being slow!
"Consent" may be one of those Roman legal terms that Easterners are uncomfortable with ;-)