The big argument against the traditional definition of marriage these days is that it discriminates against those who want to marry someone of their own gender. Those of heterosexual preference can marry anyone they want, runs this line, while those of homosexual preference can’t; this, it is asserted, is discriminatory.
Leave aside that this isn’t necessarily so as a matter of fact (prohibitions on bigamy/polygamy, marriage of siblings, etc.), and let’s consider it as a matter of logic. Discrimination in law is generally understood to refer to situations in which the law is actually different for different groups. Pale-skinned people are allowed to vote, but people whose skin is dark, or who are known to be related to anybody whose skin is dark, are not allowed to vote. Male adults are allowed to vote, but female adults are not. People who have never been convicted of a felony are allowed to vote, while those who have a felony conviction are not. The law defines groups of people and explicitly extends rights/privileges to one which it denies to the other.
On this standard, is the traditional definition of marriage discriminatory? No. It does not define groups of people, nor is it applied unequally; it is one common standard which applies to everyone. The law does not say, for instance, that “straight people” can marry people of their own gender, but “gay people” can’t; that would be, inarguably, discrimination, because the marriage law would be different for different legally-defined groups. It simply says: this is marriage; within this definition, do what you will.
Why then the accusation of discrimination? Because the traditional legal definition of marriage forbids everyone to do what only some people want to do—thus the restriction is felt as a meaningful limitation by some people but not others. “You can do what you want to do, but I can’t, and that’s not fair.”
That may sound reasonable, but consider: that’s true of every law; by this standard, every law is discriminatory. Laws against drug use discriminate against addicts—I can put whatever substance I want into my body, since I have no desire to take anything illegal, but addicts can’t. Laws against polygamy discriminate against those who want to enter into multiple marriage—they don’t restrict me in any meaningful way, since I have no desire for more than one wife (I agree with Rich Mullins on that one), but those folks clearly aren’t free to marry whomever they want. Indeed, even laws against discrimination are discriminatory; I’m free to hire whomever I want, and I’d be free to rent to whomever I wanted if I had anyplace to rent out, but racists aren’t. It is the nature of laws to discriminate against those who want to break them.
Now, if that’s a form of discrimination, you need to realize that it’s a form which is not only defensible, but necessary—logically, intrinsically necessary, if there is to be any such thing as law at all. Laws draw lines, it’s just what they do. If you want to argue that a given line shouldn’t be where it is, by all means go ahead; but don’t argue that the mere existence of the line is unfair. When once you start doing that, you’ve started cutting a great road through the law just for the sake of getting your own way; and as Robert Bolt memorably had Sir Thomas More argue, that’s a really bad idea.