I have a hitch receiver on my sedan. I've been tempted to get a set for it because I think it'd be funny.
Edit: I've also thought about taking it a step further. My car also has a wiring harness for trailer lights. What if I drill in some red/yellow LED's into the nuts so they'll light up when I brake or use the blinker?
They're illegal in my country county, but I have seriously considered seeing if I could find a vagina to go there instead, since they're not covered in the law.
I'm pretty sure they're illegal where I am too. I think they were considered obscene or vulgar. To be fair, some of them are made to look exactly like a scrotum. That might be too far.
They're illegal in my state too, but they based it on already existing traffic laws regarding distractions. For example you can't have colored lights that illuminate the road under your car.
Also, we're not allowed to have anything hanging off vehicles externally in case it comes off and causes an accident.