…it seems that you are saying that if the graces received (which I think may be the case) from the Eucharist were lifelong then it would not be a sin to receive Him in a state of sin. This doesn’t make sense to me (but again I am grateful for some aspects to consider).
“Oh happy fault, oh necessary sin of Adam, which gained for us so great a Redeemer!”
(from the Easter Vigil liturgy)
Just because God can bring something good out of an action doesn’t mean the action itself was not sinful. It just means that God remains good, and seeks us out even in our sinfulness.
The Eucharist, of course, wipes away venial sin. If we approach in mortal sin, then our actions will condemn us. If only we hear the condemnation, though, soften our hearts and return in repentance, we will be forgiven.
When God corrects, it is always for our good, always in the interest of our redemption. Only our hardheartedness keeps us from His goodness and unrelenting love. God’s correction is nothing other than the reality of our choices making themselves felt.
A marriage entered into in a state of mortal sin (or else marred by mortal sin after vows are exchanged) is an opportunity for grace that God leaves open, if we only open ourselves to the grace offered. This lifelong relationship between the one man and the one woman is really life long. In the case of Eucharist that you mention, it is the single opportunity for communion with Christ that is lost…irreplaceable, but not unrepeatable. At each Mass (and within each marriage in which grace has been rejected, for that matter, whether before or after vows), Christ waits in longing for our return to Him, for us to hear his invitation and approach, having been turned from pavement and into soil again by His call to the Sacrament of Penance. (This could get into a discussion about the one eternal Sacrifice that each Mass returns to…in that sense, the door is always open. Only the particular opportunities are temporary. That’s not where I’m going.)
What I mean to say is that it is our hardheartedness alone that closes the doors of grace. The door of God’s love is always open. The reality of what it means to turn towards Him or away is natural, though, and not arbitrary. God’s justice is nothing other than the truth. We cannot reject Him and turn to Him at the same time. We cannot both choose love and reject its demands, as they are all necessary to love’s nature. We cannot choose life and reject love. It is impossible, because by God’s great gift, our choices matter.