If you're like me and frequent read the worst in conservative trash, you've seen this hypothetical question thrown out there by opponents of Marriage Equality: If Gays Can Marry, Why Not Brothers and Sisters? The point is for conservatives to get Marriage Equality proponents to admit that marriage isn't just about love thus lending credence to their rhetoric about the biological creation of children, among other things. Lately I've seen this repurposed as If it's about benefits, won't mothers marry their sons for those benefits?
Both of these questions are stupid. Why? Because slippery slope arguments only work if there is an actual slope, i.e. a connection between two points. The notion that Marriage Equality is going to open the floodgates for any sort of cultural apocalypse is preposterous. There is no connection between homosexuality and pedophilia, or bestiality, or incest, and to suggest that there is, is to rub your balls on the face of science, not to mention common sense.
If you're going to advocate banning something because it might lead to something else, you should probably have tangible evidence that it actually leads to that other thing.