cory
"In many eras it has also been used for the union between a man and several women. This is clearly what it means in the Old Testament. And in many parts of Africa. And to some extent in old Germanic society too. "
Well I'm not sure that polygamy and polyandry are exceptions.
Even then, marriage has still been a union between one man and one woman. But if a man had, say, 3 wives it's just that there were 3 marriages, not one group marriage. My understanding is (and I'm not an anthropologist so I may be wrong) that in a polygamous system (as distinct from, say, a harem) the women are kept separate.
By the way I'm all in favour of people having equal rights. Civil partnerships do this. It's whether when we elect a parliament we also entrust the English dictionary to them to change at will. I don't think we do.
Restricting the term "marriage" to heterosexual couples is not a matter of restricting rights, it's just preserving the language from brutal change.
The question "Should two people of the same sex be allowed to marry each other?" sounds like a similar question to "Should women be allowed into the Long Room at Lords" but they are totally different. In the latter case, there is no change in the language.
Of course if people in real life do start and continue to use the "term "marriage" to refer also to same-sex couples then eventually the usage will stick and the language will have changed.
In this country, where we don't have any equivalent authority to the Academie Française, that's the way language evolves.