So instead of just using the word marriage for something that IS a marriage, we have to invent another term that doesn't mean anything else than marriage, but of two people who are of the same sex. Just because we have to. And that isn't petty.
Shall we use 'marriage' to also cover the union of a man to a horse? man to a table? man to a child? They have civil rights too!
(In the same way, I'd even describe your reason as 'religious, just there are no gods and churches and other deities involved'. We are also used to call fanboy disputes between the text editors or operating systems as religious wars. And it's a very good description.)
I'd be happy calling my argument 'historical'. I don't like changing things that have served us well for centuries.