This might give at least a hint:
http://en.wikipedia.org/wiki/Lawrence_v._Texas#Scalia.27s_dissent
"Scalia wrote that if the court was not prepared to validate laws based on moral choices as it had done in Bowers, state laws against bigamy, same-sex marriage, adult incest, prostitution, masturbation, adultery, fornication, bestiality, and obscenity would not prove sustainable."
Sure: fornication, SSM, adultery, and masturbation are not and can not be made legally different from bestiality, and prostitution. That slope is sure slippery! I guess under the same criteria, any restriction in towns setting up speed traps along the highways would inevitably lead to abolishing murder as a crime. If you do it "for" one you have to do it for all...