It's been going for probably pushing 20 years... I remember the uproar when it came out! I personally like the show, it's evolved and remained pretty on point tackling current issues in a way only SP can.
It's up to parents if they let their kids watch adult cartoons, after so long it's not like the creators have hid what it is! Parents should know what their kids are watching (or playing)... it's easy enough to research. And 20 years on from its first series they can hardly claim ignorance to what it is...
Hardly the shows fault, anymore than kids watching adult themed films or live action. Family guy, rick and Morty, Bojack Horseman all do a lot worse than SP these days... I'd go to say it's almost tame.
The film was rated a 15 when it came out, I was only just old enough to get in to the cinema!