Let's say that there's a controversy over whether people should be required to eat Brussels sprouts or whether they should be prohibited from eating Brussels sprouts. (I know it's a false dichotomy, but run with me here.) Obviously, we'd have a big fight on our hands. People (such as myself) who find Brussels sprouts disgusting would strongly resist any attempt to force us to eat the damn things. Likewise, those who (inexplicably) find them delicious would strongly resist any attempt to force them to go without Brussels sprouts.
Sometimes, there are issues we just have to fight about. Should, for example, husbands be permitted to rape their wives? It is only recently that the law has answered this question in the negative. There are some who disagree, and we fight them: we put them in jail if they disobey, and any philosophical argument they might raise in their defense to justify their behavior is simply irrelevant.
If we look at the Brussels sprouts issue, we realize that the same issue applied to every kind of food: spinach, broccoli, ice cream (which a lot of people find vile), and on and on. If we simply let the majority rule on each individual food item, we find that the majority of people (indeed almost everyone) will be denied one food or another, with no compensation. Majority rule leads in this case to majority dissatisfaction. We must look at this issue more deeply.
(We don't have to find that majority rule leads to majority dissatisfaction to look more deeply; just the fact of a strong, deep controversy is sufficient grounds to look. But that there are occasions where majority rule leads to majority dissatisfaction justifies looking more deeply in general in powerful way.)
One way of looking at an ethical issue more deeply is to ask a more general, abstract question: how do we feel about food in general, without naming the specific foods? Can we come to a broad, relatively non-controversial agreement on general principles, and apply them in such a way that the majority is satisfied? And of course we can: we eat what we like; we are prohibited from eating (or more precisely selling) only food that will kill us quickly.
The same analysis also applies to speech.
Let me be frank: I wish the Nazis, the racists, Fred Phelps, etc. ad nauseam would just shut the fuck up, and if they did, I would be unabashedly happy. If you asked me the question in isolation — Should we force these assholes to remain silent? — I'd probably say yes.
But of course everyone has some sort of minority opinion, so answering each question of speech in the particular would lead to most everyone forced to be silent about something. (There are, as John Stuart Mill explained, additional arguments against silencing minority opinions.) So we have to ask the general question: What in general should people be permitted or prohibited from speaking? And the answer we've come up with is to permit everything except provable libel, slander, incitement to riot, treason and conspiracy.
We can ask an even more general question: what activities should be tolerated in general for everyone. More specifically, what comprises the set of universal human rights?
What California Proposition 8 has done is remove the right to marry from the set of universal human rights. It is now acceptable for the government to arbitrarily determine, subject only to the will of the majority, who may marry and who may not. And far from "protecting" marriage, they have made marriage more vulnerable.
I am married. I'm in an interracial, atheist, intentionally childless marriage. All of those are minority positions. Now that Proposition 8 has passed, now that marriage is not a universal human right, it is legitimate for the government or the majority of people to deny me the freedom to marry on the basis of any of those factors. Traditionally people marry others of their own race: I did not. Traditionally people are married by a member of the clergy: I was not. Traditionally people are married to provide for their children: My wife and I will not have children (indeed we cannot; I've had a vasectomy). (And traditionally people marry others of approximately the same age: My wife is 20 years younger than me.)
Traditionally Americans are married in a Christian church. Those who are not Christians do not. Traditionally Americans are married in a Protestant Christian church. Those who are not Protestant Christians do not. Traditionally people marry someone from their local community. Traditionally people marry someone of the same religious faith. Traditionally people marry someone at about the same level of physical attractiveness. Traditionally people marry in their 20s.
Since we have now set a precedent of answering the question of who should marry in the particular, that the majority has a right to judge this question at the specific level, everyone whose marriage is non-traditional in any way could lose their right to marry by the arbitrary will of the majority.