One justification for disallowing bans on same-sex marriage is that it's seen as discrimination to prevent same-sex couples from marrying. [In this post I'm not considering under what circumstances discrimination is wrong and when it's perfectly ok. The moral issue isn't my interest here. I'm just looking at whether it's discrimination, leaving aside the moral issue of whether such discrimination is ok. It's ok to discriminate against black people when casting a part in a play for a character that was written as a white racist. But it's still discrimination, just a perfectly legitimate kind. I'm interested in the legal implications here, not the moral ones.]
Whether a practice or act counts as discrimination depends on some assumptions. Two key issues are (a) who is being discriminated against and (b) on what basis.
Consider Loving v. Virginia, the Supreme Court case that overturned bans on interracial marriage. The Supreme Court ruled that the Equal Protection clause of the 14th Amendment prevents states from treating individuals of different races differently when it comes to who they can marry. If a man is black, he couldn't marry a white woman in Virginia, but if he'd been white then he could have. That's discrimination against individuals along race lines.
Restricting marriage to same-sex couples isn't quite parallel. It doesn't discriminate against individuals according to sexual orientation. A gay man has the same rights as a straight man. He can marry an unmarried woman who is of age or who otherwise satisfies the requirements for marriage (parental consent or whatever). Both can marry women, and neither can marry men. Similarly, a lesbian has the same rights as a heterosexual woman. Both can marry men, and neither can marry women. That's not discrimination according to sexual orientation, since people of both sexual orientations (holding sex constant) have exactly the same restrictions. The law is equally applied to gays and straights.
But it is discrimination against couples. Same-sex couples are not allowed something that opposite-sex couples are allowed. Does a couple have the kind of legal status to serve as a party in this kind of legal question? My suspicion is that it would be a major innovation in our legal system to treat a couple as a legal entity. I'm not sure that's the best strategy for same-sex couples to try if they want to make headway on this issue, but it is the easiest way to end up with a discrimination claim on the basis of sexual orientation.
I've long thought that the most promising case that bans on same-sex marriage are discrimination is to ignore sexual orientation entirely and to focus on a different basis of discrimination. Men are being discriminated against on the basis of their sex by not being allowed to marry people women are allowed to marry, and women are being discriminated against on the basis of their sex by not being allowed to marry people men can marry. If you ignore sexual orientation, as many social conservatives want to do, then this complaint gets a footing. Of course you have to think any discrimination on the basis of sex is wrong or explain why this particular one is if others aren't, which puts you back to square one if you want to draw a negative moral conclusion, but I'm ignoring that in this post.
We do have reasons to discriminate on sex lines. We're perfectly ok with men's and women's bathrooms and men's and women's soccer teams. We have lots of laws or common judicial practices that favor women in child custody and child support payment cases. Sometimes laws are harsher against sex crimes committed against women. Women are given the privilege of choice to be a parent in ways men are not, with respect to abortion laws first and foremost, even though men are held responsible for child support payments even if they did all they could do to prevent the childbirth that a woman has every legal right to prevent if she doesn't want responsibility to care for a child. Not everyone thinks all of these distinctions are legitimate (I don't think all of them are), but our legal system does in fact make such distinctions all the time. Those who support affirmative action for women also have to oppose seeing sex discrimination as inherently wrong.
It's out of wanting to preserve some of those legal distinctions that many have resisted an Equal Rights Amendment to the Constitution, since that would remove all such distinctions. I'd be happy to see some of them go, but it would be throwing out the baby with the bathwater to get rid of all or most of them, as an ERA would do if it weren't worded extremely carefully (and I'm not sure it would be easy to agree on the right exceptions and then to find some brief language that summarizes those exceptions). My understanding is that President-elect Obama intends to pursue and support an ERA. But until that happens, this argument has no footing at the federal level. Only states with equal protection language for sex distinctions can (and indeed must) require state recognition of same-sex marriage. (But they also must legally do a lot of other things that they don't do, and those who use this argument will, for consistency's sake, need to be ruthless in removing any recognition of male and female differential treatment. Otherwise it will be hard to see them as genuinely motivated by following the rule of law but rather as doing whatever it takes to win on their particular hobby-horse issue.)
But it does seem to me that any state constitution with equal protection for men and women along sex differentiation lines does have the legal obligation to recognize same-sex marriage. That may become a reason for some to oppose such equal protection, but it's not as if there isn't already a number of good reasons to oppose an exceptionless ERA. This may also become an additional reason for supporters of ERA language to favor it, although it should be clear from what I've already said that they better pursue the consequences ruthlessly if they're really committed to such language for its own sake (as opposed to pursuing it just for a few issues but not for all it implies), and I'm sure such ruthless pursuit of the implications of a blanket ERA would be unpopular.
One implication of this is that states that are basing same-sex marriage on discrimination against gay individuals are making a huge mistake (a factual mistake, that is; whether it's a moral or political mistake will be controversial), especially if the state constitution already has equal protection for women, as I'm told California does. The California Supreme Court decision that Proposition 8 has (so far) managed to overturn is thus the right legal conclusion for the wrong reasons, since it was based on sexual orientation discrimination when it could have been based on sex discrimination.
California does need to be pursue whether Proposition 8 is unconstitutional, but the main reason is because it needs to be decided whether an amendment to the California constitution can overturn a legal decision made on the wrong basis when the amendment addresses the wrong basis and not the right basis. Does Proposition 8 overturn the equal protection language on issues of sex discrimination, or does it merely overturn the argument made in the actual decision it was a response to (which isn't the real reason California's constitution guarantees same-sex marriage)? I don't have any idea what the answer to that question is, but that's what seems to me should be the most important question the Supreme Court of California should face in evaluating Proposition 8. I'll be really surprised if it even comes up, but that's often how these things go.
The superior court which first ruled same-sex marriage legal in California agreed with you that it was an instance of sex discrimination. See pp. 15-16 of the state supreme court decision, which summarizes the superior court decision.
I don't know what the legal definition of the term is, but I think 'discrimination' is often used to mean something like 'giving different treatment to a person on the basis of factors irrelevant to the matter at hand.' Generally, when we show that something like race (in the case of the actor) or gender (in the case of public restrooms) is relevant to the matter at hand, we then don't call it discrimination any more. But perhaps that's just because we think discrimination is BAD, but we don't think that what's happening in these cases is bad.