This seems like the optimal legal approach to me, too. And I'm not really sure why no progress is being made toward it: I've rarely if ever heard objection to it and a "government out of marriage" approach seems appealing to most people on either side of the gay-marriage-legality issue that I've spoken to. [Obviously this is anecdote and narrow at that.] I suppose it's just not politically possible right now — if it were to gather any steam, no doubt it'd get bogged down in "these people are trying to abolish marriage!"; and it seems likely a more difficult solution to legally execute well.

But the immediate question, regardless of whether government should be involved in marriage or not, is whether we should pass laws to cause the government to treat this institution that it is involved in in a fair and equitable way (and reject laws that would enshrine unfairness).