“I think you can see the kind of slippery slope we’re going down here,” he said. “What I see here is individuals trying to change the definition of a longstanding institution called marriage to fit into their agenda.” -- New York Assemblyman James Tedisco
You know it's not a good sign when a person uses the words "slippery slope" in a public argument and thinks that that makes his point more compelling.
Merely because something can happen doesn't mean that it will happen. Support for gay marriage is not a predictor for future support for polygamy. Indeed, I support the former and oppose the latter because one is about advancing equality while the other is about demeaning it. Allowing a man to marry multiple women demeans the women--they become another status symbol, another thing to acquire. This doesn't mention the inconvenient fact that few women in this country are particularly interested in being part of a brood of wives.
What one sees, over and over again, out of gay marriage opponents is a fear of change, pure and simple. It is, among many, a very acute fear and a sympathetic one. But that is all it is. And that is what this quote suggests: a fear that their model of society is being rejected. In reality, allowing gays and lesbians to be married will have at best a marginal effect--the other 97% of new marriages will follow the one man, one woman model. This isn't a redefinition so much as a small change to the admission criteria.