A lot of 'straights' support the rights of gays to have a 'Legal Partnership' or a 'Gay Union', but would like a different, special word for their own form of 'Straight Union', or traditional 'Marriage'.
See, no religion has a claim on the word 'Marriage', but some people would like to think that they do. While the Oxford English Dictionary calls 'marriage' a "Formal union of a man and a woman," the dictionary has never been opposed to the evolution of language.
We all know how relgion feels about evolution but unfortunately, they have no say in the matter whatsoever. Language evolves, and gays want 'Marriage'!
For Straights who would like for their 'Unions' to be somehow different from 'Gay Unions', they should be free to define their 'Marriages' however they like.
As for 'Marriage', it's a pretty gay thing to want.