Because it offers legal incentives for people who choose to engage in it. The problem here isn't a legal question, but an overly-emotional argument over the word "marriage" itself, the lequal inequities that result from legislation like this being a mere side-effect that doesn't concern those who pursued Prop. 8.
There should be be no legal incentive or disincentive for people to marry or not marry. It shouldn't be the government's business beyond the enforcement of any other standard legal contract with absolutely no special status. Again, you seem to focus on the word marriage when I'm saying you're engaged in entirely the wrong debate.
Specious arguments like this are fielded by Prop. 8 supporters (most common being Daddy and Daughter want to marry) to distract from the question at hand, mainly by painting it as opening the gateway to incest.
First off, it isn't a specious argument to ask for people who aren't in romantic relationships but otherwise cohabitate to ask for additional rights granted to those who are currently "married". I know two men who are likely never going to marry, but aren't gay and would like a say in the same aspects of their lives that a currently "married" couple would. In fact, it's the fundamental point of the debate - that marriage as it's currently defined abrogates the rights of many individuals within society. That includes those in straight, gay, transsexual, polyamorist or platonic relationships alike. Nobody is any more or less special than anyone else, so don't treat anyone more or less special. Period, no slippery slopes, no extras, just full equality.
Marriage has benefits conferred upon it specifically to discourage this. Why do you think gays want to get married?
Again, why should marriage confer special benefits if they don't want them? Did you ever think that maybe some people don't want those benefits or the responsibilities that come with them? This is why I specifically brought up the issue of states automatically deeming cohabitants as common-law married. People can ask for those rights if and when they want to. Why should they be "discouraged" in any way if they don't desire to be married? If you're of the age of majority, you choose who you want to be with and who you don't. That's part of being a grown-up, and living their life as they choose (as you yourself say below).
If the question comes up, we'll answer it (and hopefully on the side of them being free to choose how they live their life as they choose.)
It is an issue, but just not one covered by mainstream media or in the forefront of people's minds.
Sure they have, mostly by supporters of Prop. 8 to discolor and trump up fears and smear opponents. Otherwise, they're completely not relevant to the question at hand.
Show me one article for anyone on either side of Prop 8 discussing the destruction of wealth, or the negative side effects of the broader problem of divorce on children (and I don't mean those who say if Prop 8 is repealed that their families will be "broken up"). This is what I'm talking about. You're again misidentifying what I'm trying to argue - that marriage, as it's defined today, and regardless of whether it's among two straight or two gay people, has many other negative side effects that nobody seems to want to address.