Why was 6 afraid of 7?
Because 7 was a registered sex offender.
Listen up, buttercup. The joke plays on the old "Why was 6 afraid of 7? Because 7 8 9!" Now, instead of simply "eating" 9, 7 is a registered sex offender. It's funny because it replaces a silly punchline with something super dark. You wouldn't get it because you are not bright.