Indeed! We are products of our surrounding culture. The problem is that the state (i.e. government at all levels) has asserted authority over a sacrament of the Church. This forever changes the definition and popular perception of marriage. I’ll bet they did this with good intentions, but we can we see where good intentions have gotten us.
The three elements of sin apply first, last, everywhere and to every human thought, action or inaction. 1. Grave matter 2. Knowledge that it is grave matter 3. the full consent of the will. Generally, it is not all that easy to mortally sin and it cannot be done accidentally.
Those without understanding will be held to lesser account than the rest of us - assuming that we know better. As a principle of how the Lord perceives such cases, (the most saintly of us rely 100% on God’s mercy), He told the following parable:
Luke 12:47-48 "And that servant who knew the will of his lord, and prepared not himself, and did not according to his will, shall be beaten with many stripes.
But he that knew not, and did things worthy of stripes, shall be beaten with few stripes. And unto whomsoever much is given, of him much shall be required: and to whom they have committed much, of him they will demand the more."
Grant you, this is a parable, bu parables demonstrate the principles by which God acts. Since we know our God is endlessly merciful - right? - we can rely on Him showing mercy to those who “know not what they do”.