Let me first clarify that I am not talking about anyone's particular religious definition of marriage, which could be any number of things, but only about a reasonable definition of civil marriage.

I'm going to keep the focus on two people, although I will comment on the numbers issue at the end.

One could, theoretically, logically define civil marriage as a union between two persons for the sole purpose of procreation and child-rearing. However this would leave out many heterosexuals who are currently validly legally married, and they would probably object. Furthermore, it would not exclude gays and lesbians, unless it was specified that the procreation and child-rearing could only involve the biological offspring of both individuals (and this will be possible pretty soon anyway!), in which case even more heterosexual couples would be disqualified.

So, leaving out procreation and child-rearing, one can logically define marriage as:

A legal and social bond between any two persons (*) (regardless of their physiology, skin color, social background, genital configuration, etc.), existing on both private and public levels, reflecting an act of committment to a common life (establishing a common household/family unit), and imparting certain rights and responsibilities.

(*) Excluding, arguably logically, immediate biological relatives, for various complex reasons that I can't begin to fully explain.

Sure, one could word it in all kinds of other ways, but I think this is pretty good.

So, then, given this definition, it makes no logical sense to further define it as a union between two people, one male and one female, because such a union is entirely possible between two females or two males. Some people don't seem to think so, but I argue that this is only because they don't actually know any loving, committed same-sex couples, so it's simply ignorance on their part and no logical proof of anything.

The only thing that is not possible between two females or two males that is possible between one female and one male is penile/vaginal intercourse. (And all of this isn't even bringing into the equation intersex and transexual people!) Everything else is possible. It's possible for them to make a committment, to establish a household, to share rights and responsibilities, to be in love, to raise children, to care for one another, etc., etc. So then, unless we are going to define marriage as a union for the facilitation of penile/vaginal intercourse (sounds more like a marriage between a man and his bottle of Viagra!), rather than a union of two people for the purpose of sharing life together, there is no logical or legally-valid explanation for gender discrimination in the issuance of civil marriage licenses.

(QED, Amen, and Blessed Be!) ;)

Oh yeah, regarding numbers, two is logically arbitary, and I don't agree with it, but, as I've said, the vast majority of people do, and legalizing same-sex marriage isn't going to change that. Ya do what'cha can, and if it's going to be two people, then at least it ought to be any two people, not certain people's definition of which kinds of two people, whether based on race, class, religion, gender, etc., etc.