allowing gays to marry isn't redefining marriage any more than allow people of different races marry. you are simply removing a suspect class status from the issue. first it was race that was removed. gay marriage is asking that gender be removed as a factor. you are saying two people marry, regardless of race, religion, gender, etc.
polygamy redefines marriage as it brings up issues of multiples. and the debate over polygamy should have no bearing in morality but in legal issues: divorce, death, inheritance, children, etc. you can't assume that a polygamous or polyamorous is one man with individual relationships. it could be a unit where all involved love each other and believe that they should all be in a union together. the question should be dealt with from a legal stance. are there inherent reasons why we should outlaw marriages between more than two people.
but ultimately, they are distinctly different issues and there isn't a slippery slope.