There's a common wrong argument against this sort of boycott, but its comparably common rebuttal is subtly wrong as well.
common wrong argument: This is a violation of Eich's freedom of speech.
subtly wrong rebuttal: The First Amendment protects you against government infringement of free speech (including campaign donations, since money is "speech"), but it doesn't protect you against the social consequences of that speech.
While true on its surface, the reason this rebuttal is subtly wrong is that it ignores a critical distinction, which is that, as a "protected class", gays are privileged under the law. This means that you are especially vulnerable to being sued for violating their rights. As Mencius Moldbug put it in the context of McCarthyism and the anti-Communist Red Scare [1]:
"[M]ost of what we call 'McCarthyism' was a matter of 'social consequences.' Besides, the social consequences work for one and only one reason: there's an iron fist in the velvet glove. Being sued for disrespecting a privileged class—excuse me, a protected class—is not in any way a social consequence, but rather a political one."
To appreciate the asymmetry, imagine a counterfactual reality in which Eich donated against of Prop. 8 instead of for it. In this context, suppose he made a comment in the workplace about his support for gay marriage. Suppose further that some Mozilla employees, who happened to vigorously oppose gay marriage, sued on the grounds of a "hostile working environment". Such a lawsuit would have no chance of success. In contrast, in the real reality we actually live in, Eich will now have to monitor his workplace speech very carefully—one wrong word about gay marriage could be all it takes to precipitate a lawsuit against him and the Mozilla Foundation. It would be unwise to underestimate the chances of such a suit's success.
In the counterfactual universe, a pro–gay marriage Eich might still face a boycott or protests, but they would be incomparably weaker because non-gays are not a protected class. Whereas in real reality, the "social consequences" of a boycott are supported by the full power of the US Federal Government. For obvious reasons, such boycotts have a habit of succeeding.
Non-gays are absolutely a protected class. Sexual orientation is what's protected, not a particular orientation.
A business that discriminates against straight people is just as liable under the law as a business that discriminates against gay people. (This is not, as I understand it, currently illegal under federal law. Many states outlaw it, though.)
The problem is that you see Prop 8 as symmetrical, but it is not. Prop 8 is an attempt to remove rights from a group of people. Opposing prop 8 is not an attempt to remove rights from other people, but rather to grant them to everyone.
The proper analogy would be a hypothetical Prop 88 which seeks to ban straight marriage while allowing gay marriage. Ignoring the complete impossibility of such a thing ever going anywhere, donating money to support the passage of such a proposition would rightfully attract a great deal of negative attention.
Right—same with race, creed, and gender. That's the theory, at least. How fearful do you expect, say, the Super Bowl–winning Seattle Seahawks are of a lawsuit alleging racial discrimination on the grounds that its defensive squad is biased against non-blacks?
In practice, the treatment of "underrepresented" minorities vs. "overrepresented" groups is almost completely asymmetric.
Prop 8 is an attempt to remove rights from a group of people.
You can't remove a right that doesn't exist. Even if you're generally sympathetic to gay rights (as I am), the idea that the framers of the California state constitution intended to protect the right of two men (or two women) to marry each other is risible.
The proper analogy would be a hypothetical Prop 88 which seeks to ban straight marriage while allowing gay marriage.
You're assuming that gay marriage and straight marriage are equally valid relationships. This may be true, but the whole point of Prop. 8 was an explicit rejection of this premise. By positing a symmetry between gay and straight marriage, you're simply begging the question.
It's not "begging the question", it's simple logic.
If Elsa can marry Joe, but Steve cannot marry Joe, and this is purely because Elsa is a woman and Steve is a man, then you're prohibiting Steve from doing something purely because of his sex. Such discrimination based on a person's sex is wrong.
Really, sexual orientation is a red herring in this debate, but people can't properly break the situation down. Bans on "gay marriage" take rights away from everyone, not just homosexuals; although I may never exercise my right to marry a man, I should still have that right.
common wrong argument: This is a violation of Eich's freedom of speech.
subtly wrong rebuttal: The First Amendment protects you against government infringement of free speech (including campaign donations, since money is "speech"), but it doesn't protect you against the social consequences of that speech.
While true on its surface, the reason this rebuttal is subtly wrong is that it ignores a critical distinction, which is that, as a "protected class", gays are privileged under the law. This means that you are especially vulnerable to being sued for violating their rights. As Mencius Moldbug put it in the context of McCarthyism and the anti-Communist Red Scare [1]:
"[M]ost of what we call 'McCarthyism' was a matter of 'social consequences.' Besides, the social consequences work for one and only one reason: there's an iron fist in the velvet glove. Being sued for disrespecting a privileged class—excuse me, a protected class—is not in any way a social consequence, but rather a political one."
To appreciate the asymmetry, imagine a counterfactual reality in which Eich donated against of Prop. 8 instead of for it. In this context, suppose he made a comment in the workplace about his support for gay marriage. Suppose further that some Mozilla employees, who happened to vigorously oppose gay marriage, sued on the grounds of a "hostile working environment". Such a lawsuit would have no chance of success. In contrast, in the real reality we actually live in, Eich will now have to monitor his workplace speech very carefully—one wrong word about gay marriage could be all it takes to precipitate a lawsuit against him and the Mozilla Foundation. It would be unwise to underestimate the chances of such a suit's success.
In the counterfactual universe, a pro–gay marriage Eich might still face a boycott or protests, but they would be incomparably weaker because non-gays are not a protected class. Whereas in real reality, the "social consequences" of a boycott are supported by the full power of the US Federal Government. For obvious reasons, such boycotts have a habit of succeeding.
[1]: [Technology, communism, and the Brown Scare](http://unqualified-reservations.blogspot.com/2013/09/technol...)