Why can't they say that already? "Oh, maybe in the eyes of THE_STATE you're married, but not in the eyes of MY_DEITY".
Because most people recognise that they can be both religious and a citizen of a state? Seriously, what a Mormon believes religious truth is doesn't matter. What the state believes the legal truth is, does.
If the state says a speed limit is X, we understand that it is X.
Marriage as a concept hasn't traditionally been owned by organized religion,
I can only really speak to European history, but it was pretty owned by religion (not necessarily organised.) Judaism defines religion. Catholicism defines religion. Anglicanism started over marriage. Whether Eastern marriages were religious or not, a good chunk of the planet believes it is religious.
My point is that the whole "the state shouldn't be involved in marriage, but domestic unions are okay!" thing seems dishonest. State marriage is a non-religious domestic union. If they really mean "gays shouldn't be domestic unioned", they ought to just say it. If they're really concerned that the State will start interceding on their religion, why not pass a law to prevent that rather than one to stop same-sex marriage in general?
They do. Or at least a large movement does. If you're talking only about those who say "legal marriage for me, legal civil union for you", that's a fair critique. But there is a large group that just wants the government to stop having any say in "marriage" and only deal with "legal household creation". That group exists and you do them a disservice by lumping them in with the others.