Equal protection principles basically amount to the idea that individuals in similar conditions should be treated similarly. This ostensibly raises the problem, though, of determining which conditions are relevant (implying sexual orientation is an irrelevant difference).
What interests me, however, is the assertion (the crux of this kind of argument) that they are treated differently. Marriage laws typically state that one is entitled to marry someone of the opposite sex. A denial of equal protection, then, would mean that the law prohibits gay men from marrying women. I seriously doubt there is a legal ban anywhere from gays entering into heterosexual marriage. A gay man has as much entitlement to marry a woman as I do. To put it another way, I, a straight man, have equally little (in three states, equally much) opportunity to marry another man as a gay man does. ...
The only way to use the argument that there is no equal protection under the law is to, well, ignore the content of law that is supposedly being unequally applied. This is different than a black person being denied a job a white person would get. Everyone — gay or straight — is allowed a heterosexual marriage, and everyone is equally denied a homosexual marriage. ...
the argument relies on the reading of the Constitution that says everyone has the right to marry who he wants, not that everyone has the right to marry someone of the opposite sex. This is a dangerous formulation because the Constitution requires that everyone have the same legal privileges, not that everyone have a fit between his privileges and preferences.
Requiring that, in order for true equality, everyone must have a privilege that matches his preferences is too much work to ask the word “equality” to do.
Here is another variant of the same concern: the word "equality" is not a stand-alone word. It is a concept which requires a referent. Who is equal to whom, for what purposes? The gay lobby takes it as self-evident that marriage equality requires that the legal definition of marriage be genderless. Mr. Harris in effect, proposes that equality means that everyone is equally permitted to marry an opposite sex partner.
I have yet a different proposal, inspired by CA's domestic partnership law. CA permits same sex couples to register as domestic partners. California also permits opposite sex couples, over the age of 62 to register as domestic partners. In effect, this rule treats intrinsically sterile couples the same way. Why doesn't this count as equality? For some purposes, obviously, it does. We have to specify the referent, before the term "equality," has any meaning.