I'm actually against gay "marriage". I do believe in civil unions, but with a twist.
I think all unions between two individuals, whether they are a heterosexual couple or a homosexual couple should be a civil unions. If one chooses to have their civil union overseen by a church, the civil union would gain an additional aspect of also being called a marriage, however in the eyes of the law, the marriage aspect offers no additional protections, risks or benefits outside of the church.
This is EXACTLY what currently exists - except for the calling it Civil Unions part. There are two forms of marriage currently in the country.
There is civil marriage - this is the state recognized marriage. A couple can choose to get married in front of a Judge or a Justice of the Peace. This marriage is recognized legally but has no religious standing, unless a church later chooses to recognize it.
The second is religious marriage. A church can choose what couples they want to marry and what couples they don't. A church today does not have to marry an interracial couple, a couple who is not a member of the church, a couple where one person is divorced etc. etc. A couple can be married by a member of the clergy and this will be recognized by the church, but not necessarily by the state. A number of churches will perform same-sex marriages in states where same-sex marriages are not legal. The marriage is recognized by the church, but not the state.
Now most marriages that take place in this country are both. We have vested the clergy with the right to perform the civil ceremony simultaneously with the religious ceremony. So most couples who get married get their civil marriage and their religious marriage done all at once. And this has lead to the confusion.
I think the key is to emphasize that these are two different concepts and that a church will not be forced to perform same-sex marriages. The California ruling only applies to civil marriage.
The Catholics, the Mormons and others are pushing for the ban. I think an effective advertisement might be to ask how many same-sex marriages have the Catholics, the Mormons and others been forced to perform in Massachusetts and California. The answer is of course none. This ruling has no impact whatsoever on a church's ability to determine who may or may not be married in the eyes of the church.
So you see, what you are proposing is exactly what currently exists in California and Massachusetts, except rather than marriage and civil unions, they are both called marriage.