If it IS grave matter and the sinner KNOWS it IS grave matter and the sinner nevertheless INTENDS to so act, then it is a mortal sin. If any one of these three conditions is absent, it is a venial sin or no sin at all.
I am going to go out on a limb here and suspect that anyone homosexually raping forcibly a thirteen-year old or serially raping a six-year old or any other kind of rapist whatsoever raping anyone is engaged in a behavior that is grave matter, knows that rape is grave matter, and is quite unlikely to have raped less than intentionally. Likewise, anyone in authority knowingly covering up such a crime. It is something more serious than minor league shoplifting, don't you think? By the way, the Catholic distinction between mortal and venial sin is quite obviously the medieval root of the governmental distinction between felonies and misdemeanors under Anglo-American common law. See Blackstone's Commentaries on the Common Law. See if you do not agree.
man may find some pleasure in ranking sins, all sins are equal before God and have been forgiven through the work of Jesus. Not a subset of sins, all sins.