"Just so you know, I think support for marriage equality is a no brainer. The argument that same-sex marriage contradicts the definition of marriage is obviously false. We do not react to as we do to phrases like "a perfectly round square".
Nor is true that we are overly keen to preserve traditions all traditions of marriage. In the Western European tradition, it used to be that the tradition was that marriage required no consent. It used to be that marriage could be solemnized with children as young as 12. It used to be that on marriage, women lost all legal standing. It used to be that on marriage, all of a woman's property became legally her husbands. It used to be that on marriage, a woman could not dissent from sex with her husband, so that if he forced her, it was not legally rape. All of these traditions (with there associated traditional definition of what marriage involved) we have been glad to get rid of as frankly immoral. That we should now further depart from the traditional Western European custom to, IMO, amend a grave, immoral restriction is no more troubling than that we no longer consider marital rape acceptable in law and custom. Certainly, the argument that we should preserve that tradition because it is the tradition fails in the face of these repeated amendments of what was involved, by law and tradition, in marriage.
When we look outside the Western European tradition (or far enough back in time within it), the argument that we should not change the definition or marriage becomes even worse as the traditions then clearly include cases which are not "an exclusive relationship between a man and a women entered into for life" as the current law insists it is. Indeed, in traditions outside of those of Western Europe, there are clear instances of marriages between people of the same sex.
While the tradition of marriage may historically have come into existence to provide for the sustenance of children, that is no longer its primary or exclusive role. No longer primary because many children are raised outside of marriage, with no legal or social stigma. No longer exclusive because we do not require by law the annulling of marriages where heterosexual couples are unable to, or choose to not have children. Further, the argument that same sex marriage cannot be allowed because marriage is about children assumes the discriminatory proposition that they should not be allowed to have children - something I completely reject.
In short, the only good argument against same sex marriage is the religious one, and it only applies to practitioners of the religions which ban homosexuality. For members of those religions, the law should be framed such that their ministers are not required to solemnize same sex marriages, and that nor are they required to provide their religious buildings as venues for such marriages. Beyond that there is no good argument for not ending the discrimination that insists (in effect, and against the evidence) that same sex couples are not capable of the same sort of long term, loving relationships that heterosexual couples are, and which marriages are intended to recognize."
(Quoted from my face book page.)