Re: my cousin
I highly recommend that you do NOT take dope's advice. Also, dope it is not necessary to teach someone "visually." Practically all of us learn perfectly fine on our own or a verbal explanation. If he still didn't get it you can always point him to puberty101.com/jackinworld.com Showing someone how is completely inappropriate.
Jack, you should not do something you aren't comfortable with and you have every right to feel uncomfortable as most people would. As you can probably figure out allowing it once led to this happening repeatedly. Hopefully you can take that as a lesson in the future not to allow it "just once."
Now you need to be firm about this. When he asks you need to firmly tell him that you will not and that it is not open for discussion and that this will not be happening anymore. If he asks why or says oh come on let me see, tell him no you will not and that it's not open for discussion.
He's younger and doesn't have quite much maturity but that should get the point across. Just be firm about it.