Laws which attempt to define/redefine marriage, or forbid homosexuals from getting married, don't, in fact, forbid homosexuals from getting married.
Even under those laws, homosexuals could get married, so long as they are of different genders. Similarly, heterosexuals couldn't get married if they were of the same gender.
Thus, it is not correct to say that it is discrimination against homosexuals. It's discrimination based upon gender.
A man can get married to a women, but not a woman. A woman can get married to a man, but not a man.
Rather than being pedantic, I think this is significant for two reasons:
First, the constitution doesn't address homosexuality, but does address gender vis-a-vis discrimination. It seems to me that a case could be made against such marriage laws on this basis.
Second, the fact that these laws turn on the basis of gender, rather than actual sexuality, reveals that it'd be impossible to enforce a law that discriminated based upon sexuality.
The second is tricky, admittedly. Certainly you can't prove the status of any one's sexuality, if only because sexuality is not defined in an entirely concrete matter. How are pansexuals or bisexuals to be treated? What about such cases where individuals engage in sexual acts among members of their own gender, but do not identify themselves as homosexuals? It's not a subject that, at this time, permits itself for legal enforcement.
However, there is one clear way that you can prove some one's sexuality: They admit it. Thus to exploit the grey area in discriminating against sexuality, you'd have to deny it, or at least play coy. This may be an issue of principle for some people, rightly believing that they shouldn't have to hide their sexuality in order to avoid discrimination.
Nevertheless, I think it's food for thought.