"R" rated bumper stickers:

 1.  Constipated people don't give a shit.
 2.  Practice safe sex; go fuck yourself.
 3.  If you drink, don't park; accidents cause people.
 4.  Who lit the fuse on your tampon?
 5.  If you don't believe in oral sex; keep your mouth shut!
 6.  Please tell your pants that it is not polite to point.
 7.  If that phone was up your ass, maybe you could drive a little better.
 8.  My kid got your honor roll student pregnant.
 9.  Thank you for pot smoking.
10.  To all you virgins:  Thanks for nothing.
11.  If at first you don't succeed, blame someone else and seek counseling.
12.  Impotence:  Nature's way of saying "No hard feelings".
13.  If you can read this; I've lost my trailer.
14.  Horn broken, watch for finger.
15.  It's not how you pick your nose, but where you put the booger.