I feel a need to spell-out the rebuttals to the gay-marriage slippery slope arguments. [View all]
Seeing that they are on the wrong side of history, vocal opponents of marriage equality have resorted to the logically suspect slippery slop argument. Specifically, allowing two same sex adults to marriage will lead to bestiality, incest, and marriage with children. All are nonsense.
Right now plural marriages are illegal and the ones that exist tend to be coercive and involving under-age girls. It tends to look like slavery. If a plural marriage really was based on gender equality and knowing, un-coerced consent of all involved, I would have no reason to oppose it.
The whole animal thing is impossible because it would be entirely one-sided. An animal cannot comprehend marriage, let alone knowingly agree to it. Likewise, children cannot legally consent because the law protects them from their inexperience and immaturity. So cases where only one side agrees are not analogous to same-sex marriage.
As far as incest, there are strong, built-in inhibitions to seeing close relatives as sex partners. Those inhibitions are codified in legal prohibitions. Part of the reason is genetic, but part of it is so that people can exist in families without being seen as potential sex partners by their close relatives. So, that really is not a good analogy either.
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