Comparing gay marriage to pedophilia, polygamy, and incest is homophobic, right?

However, don't we often hear that the proponents of "traditional" marriage offer a selective history in which they conveniently forget how the definition of marriage has changed over time? What about the patriarchs of the Bible who enjoyed multiple wives? Society has a right to evolve and redefine its institutions.

I actually think that is a valid argument. But, if the definition of marriage is fungible and the product of community consensus, why is comparing same-sex marriage to other currently out-of-favor marriage variants a sign of bigotry? That is, why isn't gay marriage in the same category as underage unions, polygamy, and marriage between siblings?

Let's break it down:

Intrinsically Unique.

1. Any honest broker will admit that marriage between a man and a woman has a "natural" and historical advantage in this debate. Since time immemorial, societies have recognized this form of human relationship for reasons that are too manifest and lengthy to explain in this space. Hetero marriage is fundamentally unlike same-sex marriage.

2. I did not mention this category in the above listing, but let's dismiss bestiality as an unkind and irrelevant comparison. Bestiality belongs in a completely separate category of cross-species commingling as well as an abuse of animals. Equating homosexuality with bestiality is insulting and not at all constructive or illuminating.

3. Pedophilia, sexual engagement with children, is also a false comparison. Why? The question here revolves around our definition of children. If we, as a society dedicated to human rights, are to eventually rest our case on the concept of "consenting adults," underage marriage becomes a question that revolves around the age of consent rather than "unnatural" attractions. Less different--but not comparable.

Fair Comparisons.

Polygamous and Incestuous marriages, on the other hand, entered into between consenting adults, seem fully comparable to same-sex marriage.

Why would a proponent of same-sex marriage take umbrage at this comparison?

If it is proper to redefine marriage for homosexuals, why not extend the same tolerance to plural marriages and incest?