As SL&P implies, State legislatures are trying to make it obvious that sex offenders are sex offenders by making them have a certain type of license plate that is very obvious. I don't see this working. If someone intended to do harm he could easily leave the car at home or steal a different plate or borrow another person's car, or, or, or . . .
If you're going to actually try to do something effective then you have to do something as noticeable and unremovable as the scarlet letter. So, will the legislators proposing the plates also propose that all convicted sex offenders have a 2" red bar tattooed on their foreheads and bright yellow letters "PERVERT" in it?