I don't totally agree with the Christian doomers here but there's some good points made. One common issue I have is this idea in the common discourse that "love is love" or some other nonsense that assumes love is the basis for marriage? How the fuck do you define that legally? And once you start getting into the weeds it becomes harder to argue against polygamous marriages after you've established that marriage is simply a contract of "love", and while I don't think it's at all a mainstream view we can expect that pedophiles will attempt to legalize pedophilia through such arguments as well.

Of course it's fairly easy to make arguments as to why pedophilia, polygamy, and other outlying forms of "marriage" need to remain illegal and not recognized by the state. It's much harder for anyone to argue why allowing gay couples to get marriage licenses in and of itself is harmful to society without making the slippery slope argument. The slippery slope argument I buy when it comes to the general philosophy most people have towards these issues, but it's got little bearing on the passage of any one single piece of legislation.