Usually, the slippery slope argument against gay marriage involves polygamy or bestiality, i.e., if we remove this restriction on marriage then we have to remove all restrictions on marriage. Of course, polygamy and bestiality are special cases; polygamy is associated with detrimental and extreme gender inequality, and animals are incapable of giving their consent to something like marriage. Which is to say that our existing ethical norms can both allow same-sex marriage and exclude other arrangements.
I only bring this up because of an activist in Maryland, who has deployed a novel -- and astute -- argument against same-sex marriage:
"If you pass this bill, you will set the groundwork, that one day when artificial intelligence is that advanced, we will be considering whether or not people can marry their androids."
He's absolutely right.
An android, according to Wikipedia, is a robot designed to look and act like a human. In science fiction, androids are typically depicted with human-levels of intelligence, or some form of advanced AI. Data from Star Trek: The Next Generation is a good example, as is Ash, the android character in Ripley Scott's Alien. Moreover, as a sci-fi convention, androids are often used as a way to explore the definition and limits of humanity. The central dilemma in Blade Runner, for instance, concerns the extent to which "replicants" deserve to be treated as human, with the main character -- played by Harrison Ford -- falling in love with one. What's more, this serves as one of the core conflicts in the recent re-imagining of Battlestar Galactica, as some humans debate the humanity of the androids who tried to wipe them out, while others fall in love, have sex, and even reproduce with the "Cylons."
If humans manage to create androids with advanced artificial intelligence, then it's entirely possible -- even likely -- that we'll get to a point where human-android marriage is a real question (presumably after the Android-Human Wars of 2361-2365, and the Android Civil Rights Movement of 2455-2468). With same-sex marriage as a precedent, there is no reason that we wouldn't allow this form of human/non-human union. After all, androids would be fully autonomous persons, with wishes, desires, and comprehensive conceptions of the good. It's in the nature of things that humans and androids will fall in love, and want to codify that union. If personhood is something more than biological humanness -- which, as we're realizing, is the case -- then androids would be entitled to exercising their rights as legal persons.
This would open a Pandora's box of policy issues -- how do you calculate survivor's benefits if the survivor can't die? -- but that's a solvable problem; ultimately, the state recognition of human/android love is more important.
