There are only TWO kinds of sin, mortal and venial. There is no such thing as a ‘grave sin’. There are mortal sins of grave MATTER and venial sins of grave matter.
And no, God gave you reason.
Mortal sin is not all that difficult to commit.
‘Full knowledge’ does not mean an encyclopedic and totally complete knowledge. Do you know exactly how every part of your car, for example, was made, designed, functioned, etc? Yet you drive it just the same because your full knowledge does not have to encompass a complete knowledge of every single part; you just need to know the basics.
Full consent does not mean that you enthusiastically, under absolutely no stimulant, no ‘pressure’, tell God, “I"m doing this because I hate you god”.
You can fully consent if you have had a drink of alcohol. You can fully consent if you have smoked a joint. You can fully consent if you are feeling ‘really pressured’ by your friends on social media who are not physically present or even ‘on-line’ at the time you’re committing a mortal sin.
Grave matter is not that hard to figure out especially with the help of the catechism. It’s not ‘less grave’ if you have been ‘brought up to believe masturbation is healthy’ or if you personally think that the girl you intend to seduce is ‘probably going to be my wife one day’ etc. etc.
Please do though try to understand the difference between mortal and venial sin.
Mortal: Full knowledge (the kind of knowledge you use in everyday circumstances like mowing your lawn, driving your car, operating electronics, etc.) Grave matter: Objective evils (even if you don’t think them that bad). Full consent: the fact that you know it’s wrong, and ‘grave’, and you do it anyway; nobody is forcing you to do so.
Venial: Sins which are either not of grave matter at all, i.e., ‘white lies’ (you look great in that dress when you really think, ‘it makes you look awful).
Or sins which are of grave matter but have lessened culpability due to a true lack of knowledge of the gravity (rarer than you would think, because as a Catholic you are responsible to know right from wrong, and you have many tools and should have made the effort to learn --you generally, not ‘personally’ here). Or lack of full knowledge (rarer than you would think because again, even if you were taught by Father Rainbow and Susan from the Parish Council, you have parents, you have fellow Catholics, you have catechisms, internet etc to educate you, and you should be educated). Finally, lack of full consent: Again, rarer than one thinks because full consent means pretty much that even if you were feeling slightly buzzed, or were really hormonal, or were feeling pressured because “everybody I know is doing this, and if I don’t, they’ll make fun of me”, that doesn’t mean you don’t freely consent if you decide, "I want this anyway’ even if you aren’t specifically thinking of God at the time.
