Right there
is the riposte to the 'slippery slope' argument.The mistake Greenfield makes here is equating homosexuals/same-sex couples a legally recognized class of persons in the context of ones 5th Amendment right to individual liberty with polygamous and incestuous couples, who possess no such right. A homosexual exists as a gay man or lesbian woman whether married or not, the same is not true for a polygamist or those seeking an incestuous relationship. Wishing to marry ones sister is not a protected individual liberty, nor is wishing to have more than one wife.
Perhaps you can explain how simply declaring that some people are not covered by the 5th Amendment makes it true.
The 14th amendment makes it ok to screw people if you apply due process.