This is probably a suggestion that's been made millions of times by millions of people - I don't really follow these debates, so I don't know. But, right. If the opposition to marriage is broadly based on religious lines, can we not make "marriage" an exclusively faith-based thing? So it's up to the church. Atheists, agnostics, and other non-believers of any gender combination can get civil partnerships, and make it legally binding, with perks to match. Christians can unite in the eyes of God.
So Christian "marriages" take place in a church, under the eyes of God, and the church decides who's allowed to marry based on their interpretation of their religious text. Secular "weddings" have nothing to do with God nor the bible, but are still a legally binding declaration of companionship, partnership, which broadly take the same structure as other ceremonies of union with aisles and guests of honour and kissing the bride and so on and so forth.
Because as an atheist, (or agnostic, depending on which day of the week it is, but today is an atheist day,) I would be totally fine with the church not recognising my wedding to my wife as long as my friends, my family, the state and the government recognised it. So the party's different, and you're not claiming to be together in the eyes of God, and we might not use the word "marriage," but the law treats you the same.
Probably loads of loopholes I've missed... but would that satisfy you, Omega? And other agnostics, and Christians, and particularly any gay readers?