As I said when I first posted, there is definitely a cultural thing here
First, clearing things up, I did not mean to say that nobody has ever used marriage in a religious way. Simply that when I hear "marriage", I don't think of religion. I'm aware that some people use it religiously, but I was in my teens by the time I realised it was the case, which in my opinion means it cannot be as prominent as you make it to be.
As for "husband" and "wife", I have definitely used the words often to mean "long term partners", for me/my partners and other people. Mind you, this was all in French, when the word "partner" would seem extremely weird used for romantic attachment (it can be use for sexual partners, but usually implies a lack of emotional connection, the word "partner" implying that sex is treated as an activity in the same way that a sport would be, for instance).
Once again, this is cultural, and I'm sure other countries, even French-speaking countries, will have different connotations with different terms.
I have also lived in Canada, where living with someone for a whole year while in a relationship with them makes you legally married (common-law-marriage is what they call it), and you have pretty much the same rights, and you're referred to as spouses and/or husband(s) and/or wife(ves) (but it's limited to a union of two people, and since you signed no contract you can't define the terms specifically).
I understand that you feel strongly about the issue, and that to you the word "marriage" has such strong connotations that you want to stay away from it for good. But I still disagree with your opinion that it's only or even mainly a religious word.
I knew someone who insisted that "single" meant "unmarried". It's not going to stop me from using it for people who are in no romantic relationship, and not using it for unmarried people who are.
I know people who think "feminist" is an insult, means "sexist towards men", and want to stay away from the word and use others. They're free to do so, but it won't prevent other people from using the word "feminist" in a different way.
Yes, big changes cause paperwork and hassle, and that's fine because it's a small price to pay for a big, positive change. From my point of view, though, this would be a price to pay for at best a neutral change (just a change of words) and at worst a negative one (telling religious people this word is now theirs, and preventing secular people from being able to use it, even though it's a very common word that is and has been used in secular contexts regularly).
To me, an unfortunate consequence would be that people who are used to the word and don't want to bother with a new one, as often happens (and as would be the case of pretty much everyone who doesn't feel strongly enough about it to consciously change their speech and habits) will keep saying "marriage", but now it will be said in legal texts that they must be talking about religious marriage, and therefore religion will look more prevalent than it actually is, and people will have to constantly correct themselves and said "I didn't mean it in a religious way".
I also can't help but think that people will have a hard time getting used to asking "Would you enter a civil union with me?" rather than "Would you marry me?"
I certainly know which phrasing I'm going to use when I propose to my boyfriend. (Or will "propose" have to be changed into another word as well?)