Derived from the conversation I posted with Winter, below.
(Also, I've noted biromantic tendencies in myself, so don't shoot me.)
1) For all the bluster on all sides of the aisle on "the definition of marriage", the definition is theoretically a very simple one: "a legal contract [traditionally considered between a man and a women, but that's subject to change] acknowledging the existence of a relationship, and restricting the terms of said relationship for the benefit of a third party", that third party being the government, the tribe, the future children, etc.. There's a metric butt-ton of detritus over tax breaks and visitation rights that have accumulated around this definition by various laws, but the essential core of marriage is the part I just put in quotes.
2) Having said THAT, the very term "gay marriage" suddenly becomes something of a contradiction. Implicit in the "gay" part is an EXPANSION of legally recognized relationships, expanding the choice of who can be in a consenting relationship with whom. Implicit in the "marriage" part is a RESTRICTION on the terms of such a consenting relationship; in a religious society, this may include absolute monogamy or certain divisions of care of house and home, and in more legal societies this includes splitting property ownership rights and debt obligations in a certain manner BY LAW. Thus, while I can support "gay marriage" viscerally, as an expansion of freedom, I cannot support it "intellectually", or speculate how it could be implemented, because the very term simultaneously implies an EXPANSION AND A RESTRICTION on the nature of relationships!
(It's possible that a significant number of gay couples feel the same way, as this study suggests:
It notes that, in all those countries and states where gay marriage is legal, very few couples actually take up the offer; perhaps the rest see marriage as an unwanted restriction on their relationship?)
Update, 9/2: After much conversation with Winter, again, it's noted that there is no contradiction if what is being discussed is the CHOICE of marriage, rather than the actual marrying, such that the couples can decide for themselves whether they like the chains or not. Fair enough, but once those advocates shift to suggesting the actual CONTRACT is a great win for their freedoms...see above.
She also said, and I quote: "clear definitions are luxuries ^_^" and that engies and scientists, in particular, need to dodge this. One, look who's talking, sound techie! Two...eh, another post.