Marriage

Someone was playing devil's advocate, and here states an argument that he's heard many people use about whether we should allow gay marriage.

Obligatory Slippery Slope Argument #1

If you break with traditional marriage, where can it lead? Can the same arguments justifying same sex marriage be used to wed three or more people? Three people and a large furry animal? A man and his sex toy (android)?

What is wrong with that?

There is no well-supported justification for the promotion of marriage, other than religious beliefs. I see no reason why arbitrary religious decisions should govern society, since we're not theocracies and the US specifically guards against such interference in public affairs.

Marriage could be said to promote child welfare, except that children grow up best in extended families with large support networks, not in 'nuclear' (two-adult) families. So we're not promoting child welfare: the focus on two adults rather than many adults is detrimental to child (and adult) welfare. People need more than one social contact to live healthy, balanced lives.

So marriage cannot be said to be best for the children (although it's not worst, either, since two adults are better than one). It has no practical worth. It is simply an artefact of religious dominance of our societies. Were we truly progressive we would discard it and promote extended support networks for children and adults.
However, the concept of devotion to one other for your whole life has such a cultural resonance in our societies that the emotional reaction to discarding marriage would over-ride rational argument. It doesn't matter that throughout history humans have typically remained with a partner for (on average) 7-10 years, and that lifelong partnership is unnatural and difficult for most people.

Marriage is sacred in our culture, even to many who do not acknowledge the existence of God. They still demonstrate the responses of those for whom things are sacred when you talk about marriage. Marriage cannot be questioned; it is an 'elephant in the room'.

The fact that if we deduce certain principles about marriage from current arrangements we find some things repugnant (not allowing gays to marry) shows that the current system is flawed.
When we try to adjust it to take into account more fundamental principles, such as equal rights, we find, as I have quoted above, that the adjusted rules of marriage lead to absurdities. This is because marriage itself is an absurdity.