HomeschoolDad:
We need to get rid of all this “grave” and “serious” business, and just say “mortal sin” or “venial sin”.
We are not in the business of determining when somebody else has committed “mortal sin”. That’s between himself and his confessor. We can tell the person he committed grave sin, but mortal vs. venial is not our call to make for anyone other than perhaps ourselves; even when deciding for ourselves we do not always judge correctly. I’ve gone to very orthodox priests all set to confess my mortal sin and gotten told the sin wasn’t mortal or it wasn’t a sin at all.
A lot of the differentiation has to do with whether we actually meant to commit the mortal sin, and whether we actually knew it was a grave sin when we were doing it. There is a whole heck of a lot of stuff I was confused about when I was younger for various reasons, and it wasn’t a simple matter to go find out the “right” answer especially in the pre-Internet, pre-CCC, pre-Theology of the Body days, when many adults were also very squeamish about discussing sexual behavior with teens.
I have to say that I personally don’t find the “mortal” vs. “venial” sin demarcation to be very helpful to me personally. It seems like that would only really matter to someone who was trying to get out of having to go to confession. If you go to confession regularly, and you avoid anything that seems grave and/or confess it right away when you need to confess it, then you never have to think about whether you committed a mortal or venial sin, except maybe for refraining from Communion if you’re not positive. I don’t sit around classifying my sins. I just try to avoid all sin, period.