|Laws against same-sex marriage aren't really discrimination or denial of equal protection, for the simple reason that the rule is the same for gays and straights alike: if you want to marry, your partner must be of the opposite sex.|
The obvious rejoinder will be that straights don't want to enter into such marriages while gays do. The answer to this in turn is that any law, trivially, falls differently on different people: some are breaking it, others would if they could get away with it, and yet others just aren't interested. This is true of good laws (against killing) and bad ones (against alcohol consumption). Closer in to the case in point are laws against marrying underage partners, immediate family members or people already married. Not many people want to make such marriages, but nobody calls them victims of discrimination if they do.
The classic case of a discriminatory law is one which expressly distinguishes between one kind of people and another. Christians can own real estate but Jews can't, or whites can go to white schools but blacks can't. Such explicitness isn't a necessary condition, though. Some laws manage to avoid admitting what they're about but, seen against their historical background, do this de facto. San Francisco once had a law requiring one kind of business license for laundries that delivered by carriage and another, more expensive, for those that delivered on foot. It made no mention of the fact that white-owned laundries tended to use carriages while Chinese-owned ones tended not to. The Massachusetts law that Romney recently exhumed to keep out-of-state gay couples from marrying was, according to last week's Economist, a progressive-era measure intended to keep mixed-race southern couples from coming there to evade their home-state prohibitions. So, similarly, with the Davis-Bacon act and so on and on. No one could plausibly claim, though, that laws against same-sex marriage were passed ad hoc to make life harder for gays.
This is not to say that the restriction is a good idea, simply that this particular line of objection isn't as potent as you might think.
(Problem: if, as the Supreme Court ruled decades ago, laws that say your partner must be of the same race constitute denial of equal protection, even if the rule is the same for everybody, doesn't this follow for the rule that your partner must be of the opposite sex? The answer might be that neither amounts to unequal protection, but they are bad ideas for other reasons.)