“Equally worse?” I have asked these questions as a kid: Which is worse: Killing as a soldier or killing as a cop? Stealing because you have no money or stealing because you really want something? Lying because you are protecting somebody or lying to protect yourself?
I guess we notice that a judge in a court determines the severity of a crime and its punishment and we are hoping that God, our Final Judge, will take the circumstances, details, and other related things into account for us and our loved ones on our judgment day.
There are so many levels to your question based on further details: Who are these people? Who raised them? Who helped them with their spiritual formation?
It feels like your question sets up a leniency in judgment for the soon-to-be-married couple but here are a few questions:
How does the couple know that the marriage will actually take place? Someone might break off the engagement. What is an engagement for? A test ride? Or a discernment of whether this is the person with whom to enter a sacramental marriage?
If one or both can justify breaking this law of God before marriage, what is to stop them from breaking another of God’s laws after marriage?
Why do you care if someone’s (or ones’ ) sins are less or more of a sin? A sin is a sin. Some “great” sinners have redeemed themselves through great sacrifices and actions while some “lesser” sinner may just stay stuck on the same dull note for a lifetime. I think I just want to stay away from sin as best as I can and hope never to lose a sense of what is or is not sin (things get so turned around these days).