I have always advocated civil unions, as opposed to marriage. As far as the state government is concerned, it should be a civil contract between two legal aged adults, and not restricted in any other way. If a daughter and mother want a civil union, they can do so, and the tax benefits of a current 'married couple' could be realized between them. The same with insurance beneficiaries, hospital and medical decisions, and even adoption. Where we currently allow 'married couples' to do something, we would allow civil union partners to do it instead.
This serves several purposes. It removes both the 'sexuality' aspect from state sanction, and it removes any and all 'religious' aspect as well. It allows religious institutions to maintain sanctity of traditional marriage, and it also allows 'gay couples' the opportunity to obtain the benefits they desire. As an added bonus, it helps many struggling families who have to cope with an aging parent, or siblings who depend on each other to survive, as they could 'combine assets' and benefit with taxes and such.
I have presented this idea here on this board before, and it seemed to be soundly rejected by the "gay marriage advocates" for some reason. My assumption, based on their comments, was the fact it wouldn't allow them to adequately trash religion and religious customs. Furthermore, I think the left doesn't really want to settle the issue, because it's like racism, abortion, and entitlements, it delivers a certain 'enraged' segment of voters each election cycle. Having the issue benefits them more than solving the issue.