The real mistake was for governments to get involved in marriage in the first place. If government hadn't given special privileges to couples who contracted with the state to be together we wouldn't have all these stupid arguments and gay people could get married in their church that supported that and straight people could get married in churches that supported that. And atheists would just decide to be together. The state would treat them all the same; as individuals. Certainly today there is no compelling reason for the government to be involved.
If it was left to religion, then there would be
no marriage equality. Marriage is inherently a civil matter - you can get married without religious involvement (secular wedding), but you cannot get married without the approval of the state. Those child-bride weddings in cults are illegal, even if they're sanctioned by the religion.
Religious organisations have been delegated the power to act on the state's behalf, in order to integrate the couple's beliefs into the ceremony (remember "by the power invested in me by the state of Somewhere"), but are not the only ones, and do not have ownership of the marriage process (which is why religious arguments against equality are just plain invalid).
If you want to separate religious unions from any state sanctioning, then I'm down with that. We'll call the civil ones "marriage" (as they are today) and the religion-only ones something else ("Godly Unions" or something). The government will therefore give legal, tax and financial recognition only to the civil-only marriages, since the government is the civil body. The religious organisations can give their religion-only unions whatever benefits they have available to them. Holy water and crackers, I guess.
Handing marriage over to religion scares the living crap out of me. The old ridiculous chestnut of "if gays can marry, then it's a slippery slope until people are marrying children, animals or inanimate objects" is avoided by the requirement for the participants to have legal standing to agree to a contract with informed consent - animals, children and inanimate objects cannot agree to a contract. But put religion solely in charge of marriage, remove the civil requirement for legal consent and agreement, and it comes down to what that belief system feels is okay. Such as:
So, you want freaky, sick unions that do not require legal consent? Then by all means take the state out of it.