Seevral pages ago, I know, but...
Does marriage have a definition, or is it just what society says?
Marriage is a social institution, so it is what society says it is. Or, in this case, it is what society defined it as however many years ago.
Take note that I'm only referring to civil unions; religious marriages are a whole other thing. I do support gay (religious) marriage in general, but do not beleive that any government should force any religion to perform them. The ideal of separation of Church and State aside, the idea of government dictating terms to religion just leaves a bad taste in my mouth, and that's coming from a deist who believes that the Roman Catholic Church is horrendously out of touch.
Also, technically, even if it is just what society says it is, it'll still have a definition.
If the latter, is it inconceivable that "People" will be taken out of the definition?
Re: the whole 'in the future beastiality might be legal' argument, I firmly believe people should be making decisions on laws based on current ideas and morals, without worrying about possible changes in social mores.
The main reason being that you'e making laws for people living under current moral codes, and the secondary reason being that social changes are bloody hard to predict.