I believe St. Thomas Aquinas said that gluttony, for instance, can almost never become a grave sin. So, the thinking is that if the category of sin is mentioned in the 10 commandments, that puts it inherently into the CATEGORY of grave sin ... but then most moral theologians tie just about every sin somehow to the 10 commandments. So, for instance, the 8th commandment says not to "bear false witness", but then lesser kinds of "lying" are also considered tied to the 8th commandment. I find it a bit circular there, and I'm not convinced. So, for example, answering "no" to "do I look fat in this dress?" is not, IMO, really in the category of "bearing false witness" (sending someone to jail through false testimony). In any case, this is just an attempt to rationalize or categorize stuff, and the important thing is to understand specifically what are grave and what are venial sins.
There have been some here who say that the distinction between mortal and venial sin is not important, because we should avoid all sin, but I would have to disagree that it's not important. Not only does it help to understand the principles involved by calling out what can turn a venial sin into a mortal one, but it also helps souls make good confessions and helps them to understand when they may approach Holy Communion after a sin, etc. Church has always tried to educate the faithful about venial vs. mortal sin.
Not at my son's school, they didn't.
A bit of backstory here: when my son started in school, I was working very long hours, and the greedier my employer got (translated: give me more work instead of hiring the extra people I pleaded with them for years to hire), the longer the hours got. On top of that, I was forced by court order to render a fairly substantial amount of visitation, so my son (I suppose I should really say "our" son) had to go stay with his mother and her illicit consort as well, and have to witness their adulterous living situation. I reasoned, well, the school is not perfect, but homeschooling isn't an option, I'll keep an eye on things, and fill in any of the gaps that may emerge. That didn't work. When I finally took my son out of the school, when homeschooling
did become an option, we launched right into the Baltimore Catechism #2, and there was lot of damage to undo, no rank heterodoxy, just massive gaps in his knowledge. We continued with BC #2 (Grade 8 last year was a bit of a hodgepodge, not least because my father was dying and we ended up having class when we could, many times during a late-night meal in the car at McDonald's!) and we have begun BC #3 this year. Just today my son initiated a description of purgatory that, while a bit fanciful --- he has a
vivid imagination! --- was entirely orthodox, and I used this as an opportunity to read the story of St Catherine of Genoa as it appeared in a spiritual manual posted here on CI a few weeks ago.
So where am I going with this? When we got to the part of the BC about mortal and venial sin, I just got this "huh?" look from him, and I said "they didn't teach you any of this, did they?". He said no, all they did was leave him with the impression that all sin was mortal sin, though they didn't use that term. He was scared to death. There has been a lot of correction to do on that count, and now he understands.
Moving on directly to your comments, I would just instinctively classify gluttony under the Fifth Commandment, i.e., a doing of damage to oneself through overeating. In all of my study of catechism throughout the years, I have always just taken for granted that there are sins that are not bad enough to go to Hell for, and sins that are. Broadly speaking, the former would be the lighter or more trivial (but no sin is "trivial") kinds of sins, as I described, a trivial lie, "pilfering" of a small amount of something, general grouchiness or impatience with others (I find myself guilty of this sin a
lot), grumbling or murmuring out of exasperation, and so on. As I noted above, the Sixth and Ninth Commandments (carnal sins) cannot, in themselves, not be "grave matter", if they consist of deliberate, intentional acts with the two other conditions of sufficient reflection and full consent of the will. They are a special case, and it is not for nothing that Our Lady of Fatima said that the sins that send more people to Hell than any others are the sins of the flesh --- they are the hardest to stop once becoming habit, they are the ones that most easily blind the sinner to their malice, and they are the ones easiest rationalized.