I know I keep coming back to this but I’m frustrated here. Where are you getting this? There definitely are situations where a sin against any of the commandments could be serious in one way or another, but b/c a sin is a sin against one of the commandments does NOT make it immediately mortal. Do you understand what we’re saying when we say “serious”? It doesn’t mean that a venial sin would not be serious in the sense of the word in today’s culture, but when we say serious we are meaning a mortal sin that would result in loss of sanctifying grace.
A high school kid who says “oh my G–” out of habit, especially when trying to kick the habit, without thinking, is definitely commiting a sin. I think they would be showing a disrespect for God, an insensitivity to God, but contempt? And a mortal sin? Definitely not! I’m not trying to say that committing this sin is okay, but in most cases it is going to be venial, not mortal.