USS Clueless looks at whether the California initiative making gay marriage illegal is constitutional by California's own constitution. Here's the relevant section:
(b) A citizen or class of citizens may not be granted privileges or immunities not granted on the same terms to all citizens. Privileges or immunities granted by the Legislature may be altered or revoked.
My bold claim: all the rhetoric about rights possessed by straight people that gay people lack is completely bogus. However, this statement might give California a reason to overturn the voter initiative banning gay marriage (at least among citizens) on other, really surprising, grounds -- that it's gender discrimination! However, the relevant kind of discrimination is only illegal in states with equal rights for men and women, which California has. However, by the end I'll give enough reasons to think those states should remove such language from their constitutions.
Keep in mind also that I've said in other posts that I think much of the rhetoric on this issue is overheated, even hateful (and that's the case on both sides). I don't see why Christians prioritize this issue above others with more serious consequences. I don't understand why it's the end of the world if something Christians believe is wrong becomes legal, since that happens all the time. I don't get why this small step is supposed to be such a huge modification in the concept of marriage when the huge gulf between the Christian concept of marriage and the one understood by most Americans makes this seem quite small. So take all I say in light of that. My views on this aren't as simple as you might have thought.
Please bear with me on this, since a real analysis of the arguments is going to require Chisholming away at the various options for framing the issue before we get to the punchline. These options all do get mentioned by someone or other, so it's worth looking at all of them.
The claim of the movement to legalize gay marriage is that there's a privilege being granted to heterosexuals that isn't given to gays -- the privilege to marry. Now this isn't strictly speaking true. Gay people are given exactly the same right as heterosexuals -- to marry someone of the opposite sex. So there isn't a privilege that I have that a gay man doesn't have. Similarly, all women have the right to marry a man, regardless of their sexual orientation. When framed this way, the issue seems quite obvious. There's no discrimination going on at all.
But not so fast, says the movement to legalize gay marriage. There is one privilege given to heterosexuals that isn't given to homosexuals -- the privilege to marry someone of whatever sex they want to marry. Gay people can't marry people of the sex they want -- their own sex, whereas straight people can marry people of the sex they want -- the opposite sex. But even this way of framing the rights is inaccurate. I don't myself have the right to marry someone of whatever sex I might want. So in that way my rights are the same as those of someone who is gay. We both lack the right to marry whoever we might turn out to want to marry (ignoring our actual desires). The same restrictions in fact apply to both of us, in these terms anyway.
The only way to describe a right that heterosexuals have that gay people don't is as follows. I have the right to marry someone of the sex I happen to have desires for. That seems right. There's a right both I and a gay man have -- to marry someone female. I happen to have desires that lead me in that direction (not that I'm interested in bigamy, but this is an in principle discussion), and he doesn't have those desires. Yet it's still the same set of rights that we each have. Simply describing a right in terms of whether you have the desire to achieve what that right allows doesn't make you any more deserving of that right and doesn't make it an issue of equal rights. If someone comes along and wants to marry a tree, does it mean that this person lacks a right that I have, simply because her or his desire is really unusual? How is it any different with a group that's more common but still not in any sense the norm?
On a simply theoretical level, someone's having a desire for something that he or she has no right to (and neither do I), when my desires are for something I do have a right to (and so does the other person) means that we do in fact have exactly the same rights. We just have different desires. Having desires for something you don't have a right to does not mean you have lesser rights. That's just not how rights work.
This issue is often compared to racial intermarriage, but the analogy doesn't hold up for exactly the reasons I've been outlining. With interracial marriage bans, white people had the right to marry white people, and black people had the right to marry black people. White people didn't have the right to marry black people (and it thus follows that black people didn't have the right to marry white people). This is independent of anyone's desires. Even if I'd had no desire to marry a black woman, I still would have been barred from doing so, and that would have meant that any black man had a right that I lacked, simply because of my race. Black people at one point lacked the right to marry white people, regardless of their desires to do so, and therefore they lacked a right that white people had. This is race discrimination. A right is given to one group (to marry whites) because of race (they're white themselves). This indeed violates the equal protection clause.
Now consider the charge of discrimination according to sexual orientation. The parallel would be if gay people were prevented from marrying straight people (which has happened many, many times and is perfectly legal) while straight people were allowed to do so. That's not what's going on here. There's no single right that a straight person has that a gay person doesn't have. A straight person wants to marry someone of the opposite sex and has that right, while a gay person wants to marry someone of the same sex and lacks that right. The same right that's lacked by the gay person is also lacked by the straight person. The same right the straight person has is also possessed by the gay person. This is no violation of equal protection (not that there's federal equal protection along sexual orientation lines anyway, though the excerpt I quoted above of the California constitution does seem to have something like it among citizens).
This is where I thought USS Clueless was right on the money. There are all sorts of things that we make illegal that some people want to do and others don't. I don't have a right to smoke crack. I don't want to do so. Some people do want to do so. Is it discrimination against a guy who happens to have that desire because I'm allowed to smoke what I want to (which is nothing at all), whereas he isn't allowed to smoke what he wants to (crack)? That's just silly when you think about it. You could say the same about virtually anything that's illegal that some people want to do that others don't want to do. It really is quite parallel structurally.
To leave it at that would really miss one argument the movement to legalize gay marriage has available to them. It's not gays who are denied equal rights. There's a failure to give men and women equal rights. Men have the right to marry women. All men lack the right to marry men. So there's a right women have that men don't have. Similarly, all women lack the right to marry women, while having the right to marry men. So they lack a right that men have. This clearly violates the proposed Equal Rights Amendment that looms its ugly head every few years. If men and women are required to have the same rights as each other, this policy can't stand. It's a good thing for those opposing gay marriage that the ERA isn't law (though I suppose there's some question about whether the fourteenth amendment's so-called equal protection clause will have the same effect). But wait! California's constitution doesn't mention equal rights of men and women, but it does say any right one citizen has can't be denied to another. How can men and women have these different rights, then? It seems California has a legal basis for overturning the initiative that makes it illegal for gay people to marry, but it involves this roundabout line of reasoning. The reason people are saying it needs to be overturned won't work.
There's still a problem with this. California has every reason to remove such language from their constitution, for the same reasons an Equal Rights Amendment is a bad idea at the federal level. It would require forcing Smith College, Wellesley College, and any other all-female schools to admit men. It would require forcing women's soccer and basketball teams to allow men to play on them. You couldn't legally prevent me from going into a women's bathroom (in fact I see no way single-sex bathrooms could continue with any enforcement). Any laws favoring women in terms of child custody or labor in heavy industry would become illegal. There would be no more room for an absolute right currently written into law to decide whether to have an abortion when the father disagrees (though I myself think this would be a good thing, but I'm trying to give reasons an ERA supporter would be likely to agree with). Women would not just be allowed in combat but would be forced to register for the draft and expected to fight in the same way men are. Separate barracks for women would be no more as well. Affirmative action for women in jobs where women are underrepresented would be illegal. All this is implied by current California law.
So I think there is a legal basis for what people are saying. In California, there's a legal means for overturning that voter initiative. However, that legal basis is insane, eliminating legitimate distinctions and forcing women to be, in effect, men, and vice versa. It removes any sense of independent worth of women. That part of their constitution needs to be removed on moral grounds. So there no longer remains a legal basis for this claim that's backed by any real moral consideration.