At it's most basic essence, marriage is a cross cultural constraint on evolutionary mating behavior.<quoted text>
Have you ever noticed that the only ones who talk about incestuous and plural marriages are those who are against same-gender marriages?
And they call us depraved...
By the way, using your own ridiculous "slippery slope" argument, you can trace all of these issues back to the fact that the government got involved in marriage.
If the government had never given any special rights and protections to married couples in the first place then nobody--not gays, not bigamists, not anyone--would be seeking legal marriage.
We'd all just go to the church of our choice and get married.
The only prevailing interest a government has in marriage is to protect and support the sole safe birthplace of society's members.
The government has no prevailing interest in selectively supporting friendships.