Heavy Metal, wow, sounds like RorrKonn has something big planned!

Regards the ratings, haven't read it, but at a glance I see there's a whole bunch of info at the link and it includes the "R" and "X" ratings.
Click Here for Adult Ratings (for America)

Don't know if it applies to comics though, I know there's a sort of standard for comics, I think it's different.

I have a couple of really cool books about adult comics, so it's a shame they're boxed away somewhere right now, but I think a single "X" is probably what would be considered Erotica.  And due to that, I don't see why you wouldn't be allowed to show the genitals, because I'm guessing that even a basic "X" is going to be adult-based, and without allowing the gentials to be shown, it wouldn't really be adult-based.