Gay marriage right or wrong:
This idea is essentially playing with semantics.
I did not propose that there be a separate but identical marriage for homosexual relationships and that it be called "civil unions."
"Furthermore, the effects on society are bound to be minimal..."
This is the type of reasoning that the slippery slope argument applies perfectly to. Why not let any small group demand a change with little relative impact?
"...why challenge any assumption?"
I'm not arguing that law shouldn't change. I'm arguing that law developed under one set of assumptions should be blindly applied to a different scenario. Read my rebuttal to res0ndf7 for more on this line.