Incest Prevention Method

Okay, you knucklehead, this joke is playing on a super sensitive topic: incest, which is a big NO-NO, right? The 'joke' answer is a really gross, vulgar, and totally messed up way to 'prevent' conception, implying a non-reproductive sexual act. It's supposed to be shocking and offensive because it combines a taboo subject with a crude, explicit response. Basically, it's banking on you being shocked by the crudeness. You probably don't even know what conception means, do you, you big dummy?