"Ignorance of the law excuses no one." How would you explain this?

In the news today, a professor at Assumption was convicted with finality for rape. He had sex with a 17-year old student. he said they were lovers.

But the law says you can not have sex with minors (unless you are married, and the marriage has parental consent). And that it is otherwise called RAPE.

The clueless (?) professor was declared guilty, "ignorant" of that law.

Bottomline,the courts will never let you off with an "I'm sorry I did not know it was illegal" alibi. Dura lex, sed lex.