If I say something illegal during a meeting, people will notice it and will report it to the police. Then I may get arrested or fined. Why should it be different on the internet ? Provided we live in countries where freedom of speech is enacted. Of course in Russia or china it's different but surely they already have tools like that.
Dark jokes and strong opinions are example - you something filthy - let's say dark Holocaust/Nazi joke but funny in situation In group that accept it and it's ok. But if it's recorded, it'll stay forever and will surface in most unexpected moment, like job interview or some other screening by gov/corpos.
Don't say that dirty jokes should be punished in future if in given situation they were received as ok and only later someone else, not in situation is going to judge it